Method of layerwise fabrication of a three-dimensional object

ABSTRACT

A method of layerwise fabrication of a three-dimensional object is disclosed. The method comprises, for each of at least a few of the layers: dispensing at least a first modeling formulation and a second modeling formulation to form a core region using both the first and the second modeling formulations, and at least one envelope region at least partially surrounding the core region using one of the first and the second modeling formulations but not the other one of the first and the second modeling formulations. The method can also comprise exposing the layer to curing energy. The first modeling formulation is characterized, when hardened, by heat deflection temperature (HDT) of at least 90° C., and the second modeling formulation is characterized, when hardened, by Izod impact resistance (IR) value of at least 45 J/m.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/335,299 filed on Mar. 21, 2019, which is a National Phase of PCTPatent Application No. PCT/IB2017/055696 having International FilingDate of Sep. 20, 2017, which claims the benefit of priority under 35 USC§ 119(e) of U.S. Provisional Patent Application No. 62/397,949 filed onSep. 22, 2016. The contents of the above applications are allincorporated by reference as if fully set forth herein in theirentirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to AdditiveManufacturing (AM) of an object, and, more particularly, but notexclusively, to formulations, methods and systems for additivemanufacturing of an object which exhibits desirable mechanicalproperties, for example, a desirable Heat Deflection Temperature (HDT)without compromising other properties.

Additive manufacturing is generally a process in which athree-dimensional (3D) object is manufactured utilizing a computer modelof the objects. Such a process is used in various fields, such as designrelated fields for purposes of visualization, demonstration andmechanical prototyping, as well as for rapid manufacturing (RM).

The basic operation of any AM system consists of slicing athree-dimensional computer model into thin cross sections, translatingthe result into two-dimensional position data and feeding the data tocontrol equipment which manufacture a three-dimensional structure in alayerwise manner.

Various AM technologies exist, amongst which are stereolithography,digital light processing (DLP), and three dimensional (3D) printing, 3Dinkjet printing in particular. Such techniques are generally performedby layer by layer deposition and solidification of one or buildingmaterials, typically photopolymerizable (photocurable) materials.

Stereolithography, for example, is an additive manufacturing processwhich employs a liquid UV-curable building material and a UV laser. Insuch a process, for each dispensed layer of the building material, thelaser beam traces a cross-section of the part pattern on the surface ofthe dispensed liquid building material. Exposure to the UV laser lightcures and solidifies the pattern traced on the building material andjoins it to the layer below. After being built, the formed parts areimmersed in a chemical bath in order to be cleaned of excess buildingmaterial and are subsequently cured in an ultraviolet oven.

In three-dimensional inkjet printing processes, for example, a buildingmaterial is dispensed from a dispensing head having a set of nozzles todeposit layers on a supporting structure. Depending on the buildingmaterial, the layers may then be cured or solidified using a suitabledevice.

Various three-dimensional inkjet printing techniques exist and aredisclosed in, e.g., U.S. Pat. Nos. 6,259,962, 6,569,373, 6,658,314,6,850,334, 7,183,335, 7,209,797, 7,225,045, 7,300,619, 7,479,510,7,500,846, 7,962,237, 9,031,680 and U.S. Patent Application havingPublication No. 2015/0210010, all of the same Assignee.

A printing system utilized in additive manufacturing may include areceiving medium and one or more printing heads. The receiving mediumcan be, for example, a fabrication tray that may include a horizontalsurface to carry the material dispensed from the printing head. Theprinting head may be, for example, an ink jet head having a plurality ofdispensing nozzles arranged in an array of one or more rows along thelongitudinal axis of the printing head. The printing head may be locatedsuch that its longitudinal axis is substantially parallel to theindexing direction. The printing system may further include acontroller, such as a microprocessor to control the printing process,including the movement of the printing head according to a pre-definedscanning plan (e.g., a CAD configuration converted to a StereoLithography (STL) format and programmed into the controller). Theprinting head may include a plurality of jetting nozzles. The jettingnozzles dispense material onto the receiving medium to create the layersrepresenting cross sections of a 3D object.

In addition to the printing head, there may be a source of curingenergy, for curing the dispensed building material. The curing energy istypically radiation, for example, UV radiation.

Additionally, the printing system may include a leveling device forleveling and/or establishing the height of each layer after depositionand at least partial solidification, prior to the deposition of asubsequent layer.

The building materials may include modeling materials and supportmaterials, which form the object and the temporary support constructionssupporting the object as it is being built, respectively.

The modeling material (which may include one or more material) isdeposited to produce the desired object/s and the support material(which may include one or more materials) is used, with or withoutmodeling material elements, to provide support structures for specificareas of the object during building and assure adequate verticalplacement of subsequent object layers, e.g., in cases where objectsinclude overhanging features or shapes such as curved geometries,negative angles, voids, and so on.

Both the modeling and support materials are preferably liquid at theworking temperature at which they are dispensed, and subsequentlyhardened, typically upon exposure to curing energy (e.g., UV curing), toform the required layer shape. After printing completion, supportstructures are removed to reveal the final shape of the fabricated 3Dobject.

Several additive manufacturing processes allow additive formation ofobjects using more than one modeling material. For example, U.S. PatentApplication having Publication No. 2010/0191360 of the present Assignee,discloses a system which comprises a solid freeform fabricationapparatus having a plurality of dispensing heads, a building materialsupply apparatus configured to supply a plurality of building materialsto the fabrication apparatus, and a control unit configured forcontrolling the fabrication and supply apparatus. The system has severaloperation modes. In one mode, all dispensing heads operate during asingle building scan cycle of the fabrication apparatus. In anothermode, one or more of the dispensing heads is not operative during asingle building scan cycle or part thereof.

In a 3D inkjet printing process such as Polyjet™ (Stratasys Ltd.,Israel), the building material is selectively jetted from one or moreprinting heads and deposited onto a fabrication tray in consecutivelayers according to a pre-determined configuration as defined by asoftware file.

When a cured rigid modeling material forms the final object, the curedmaterial should preferably exhibit heat deflection temperature (HDT)which is higher than room temperature, in order to assure its usability.Typically, the cured modeling material should exhibit HDT of at least35° C. For an object to be stable in variable conditions, a higher HDTis desirable.

U.S. Patent Application having Publication No. 2013/0040091, by thepresent assignee, discloses methods and systems for solid freeformfabrication of shelled objects, constructed from a plurality of layersand a layered core constituting core regions and a layered shellconstituting envelope regions.

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise fabrication of athree-dimensional object. The method comprises, for each of at least afew of the layers: dispensing at least a first modeling formulation anda second modeling formulation to form a core region using both the firstand the second modeling formulations, and at least one envelope regionat least partially surrounding the core region using one of the firstand the second modeling formulations but not the other one of the firstand the second modeling formulations. The method optionally andpreferably also comprises exposing the layer to curing energy. The firstmodeling formulation is optionally and preferably characterized, whenhardened, by heat deflection temperature (HDT) of at least 90° C., andthe second modeling formulation is characterized, when hardened, by Izodimpact resistance (IR) value of at least 45 J/m.

According to some embodiments of the invention a ratio between elasticmoduli of the first and the second modeling formulations, when hardened,ranges from 2.7 to 2.9.

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise fabrication of athree-dimensional object. The method comprises, for each of at least afew of the layers: dispensing at least a first modeling formulation anda second modeling formulation to form a core region using both the firstand the second modeling formulations, and at least one envelope regionat least partially surrounding the core region using one of the firstand the second modeling formulations but not the other one of the firstand the second modeling formulations. The method optionally andpreferably also comprises exposing the layer to curing energy. A ratiobetween elastic moduli of the first and the second modelingformulations, when hardened, optionally and preferably ranges from 2.7to 2.9.

According to some embodiments of the invention the first modelingformulation comprises: at least one curable acrylic monomercharacterized, when hardened, by Tg of at least 85° C., as describedherein in any of the respective embodiments; at least one curablemethacrylic monomer characterized, when hardened, by Tg of at least 150°C., as described herein in any of the respective embodiments; at leastone curable (meth)acrylic oligomer, characterized, when hardened, by Tgof at least 50° C., as described herein in any of the respectiveembodiments; and optionally, at least one curable (meth)acrylic monomercharacterized, when hardened, by Tg lower than 0° C., as describedherein in any of the respective embodiments, wherein a concentration ofthe curable methacrylic monomer is at least 35% by weight of the totalweight of the first modeling formulation.

According to some embodiments of the invention the second modelingformulation comprises: at least one curable (meth)acrylic monomercharacterized, when hardened, by Tg of at least70° C., as describedherein in any of the respective embodiments; at least one curable(meth)acrylic oligomer characterized, when hardened, by Tg of at least10° C., as described herein in any of the respective embodiments; and atleast one curable (meth)acrylic monomer which comprises at least 5ethoxylated groups (an ethoxylated curable material) and ischaracterized, when hardened, by Tg lower than −20° C., as describedherein in any of the respective embodiments, wherein a concentration ofthe ethoxylated curable material is at least 5 weight percents of thetotal weight of the second modeling formulation.

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise fabrication of athree-dimensional object. The method comprises, for each of at least afew of the layers: dispensing at least a first modeling formulation anda second modeling formulation to form a core region using both the firstand the second modeling formulations, and at least one envelope regionat least partially surrounding the core region using one of the firstand the second modeling formulations but not the other one of the firstand the second modeling formulations; and exposing the layer to curingenergy. Preferably, the first modeling formulation comprises: at leastone curable acrylic monomer characterized, when hardened, by Tg of atleast 85° C.; at least one curable methacrylic monomer characterized,when hardened, by Tg of at least 150° C.; at least one curable(meth)acrylic oligomer, characterized, when hardened, by Tg of at least50° C.; and, optionally, at least one curable (meth)acrylic monomercharacterized, when hardened, by Tg lower than 0° C. Preferably, aconcentration of the curable methacrylic monomer is at least 35% byweight of the total weight of the first modeling formulation.Preferably, the second modeling formulation comprises: at least onecurable (meth)acrylic monomer characterized, when hardened, by Tg of atleast70° C.; at least one curable (meth)acrylic oligomer characterized,when hardened, by Tg of at least 10° C.; and at least one curable(meth)acrylic monomer which comprises at least 5 ethoxylated groups andis characterized, when hardened, by Tg lower than −20° C. Preferably, aconcentration of the ethoxylated curable material is at least 5 weightpercents of the total weight of the second modeling formulation.

According to some embodiments of the invention the curable methacrylicmonomer in the first modeling formulation is characterized by a curingrate lower than 4400 mW/minute.

According to some embodiments of the invention, a concentration of thecurable methacrylic monomer in the first formulation ranges from 35 to50% by weight, of the total weight of the first modeling formulation.

According to some embodiments of the invention a concentration of thecurable acrylic monomer in the first formulation ranges from 10 to 40%by weight of the total weight of the first modeling formulation.

According to some embodiments of the invention a concentration of the(meth)acrylic oligomer is the first modeling formulation ranges from 10to 40% by weight of the total weight of the first modeling formulation.

According to some embodiments of the invention the ethoxylated curablemonomer is characterized by at least one of a viscosity at roomtemperature lower than 1000 centipoises; and a molecular weight of atleast 500 grams/mol.

According to some embodiments of the invention a concentration of theethoxylated curable monomer in the second modeling formulation rangesfrom 10 to 50% by weight of the total weight of the second modelingformulation.

According to some embodiments of the invention a concentration of thecurable (meth)acrylic monomer in the second modeling formulation rangesfrom 10 to 50% by weight of the total weight of the second modelingformulation.

According to some embodiments of the invention a concentration of thecurable (meth)acrylic oligomer in the second modeling formulation rangesfrom 10 to 50% by weight of the total weight of the second modelingformulation.

According to some embodiments of the invention the first and/or secondmodeling material formulation further comprises an initiator forinitiating the curing.

According to some embodiments of the invention a concentration of theinitiator in the first and/or the second modeling material formulationindependently ranges from 0.5 to 5% by weight of the total weight of therespective formulation.

According to some embodiments of the invention the first and/or secondmodeling material formulation independently further comprises at leastone of a surfactant, a dispersing agent and an inhibitor.

According to some embodiments of the invention the first modelingmaterial formulation is characterized, when hardened, by heat deflectiontemperature (HDT) of at least 90° C.

According to some embodiments of the invention the second modelingmaterial formulation is characterized, when hardened, Izod impactresistance (IR) value of at least 45 J/m.

According to some embodiments of the invention the second modelingformulation is characterized, when hardened, by HDT lower than 50° C.,or lower than 45° C.

According to some embodiments of the invention the object is constructedfrom a plurality of layers, a layered core constituting core regions andat least one layered shell constituting envelope regions.

According to some embodiments of the invention there are two enveloperegions.

According to some embodiments of the invention the dispensing is in avoxelated manner, wherein a thickness of an inner envelope region of thetwo envelope regions is from about 0.1 mm to about 4 mm, and wherein athickness of an outer envelope region of the two envelope regions isfrom about 150 microns to about 600 microns.

According to some embodiments of the invention there is an additionalenvelope region between the inner envelope region and the outer enveloperegion.

According to some embodiments of the invention the additional enveloperegion has a thickness less than the thickness of the inner enveloperegion and less than the thickness of the outer envelope region.

According to some embodiments of the invention the thickness of theadditional envelope region is from about 70 microns to about 100microns.

According to some embodiments of the invention an amount of the firstmodeling formulation is the core region is higher than 25% by weight ofa total weight of the core region.

According to some embodiments of the invention the dispensing is in avoxelated manner, and wherein voxels of the first modeling formulationare interlaced with voxels and the second modeling formulation withinthe core region.

According to some embodiments of the invention there are two enveloperegions, wherein a thickness of an inner envelope region of the twoenvelope regions is from about 1 to about 5 microns, and wherein athickness of an outer envelope region of the two envelope regions is afew voxels.

According to some embodiments of the invention the core region, whenhardened, is characterized by HDT of at least 60° C.

According to some embodiments of the invention the method comprises,during the dispensing and/or the exposure to the curing energy, exposingthe layers to heat.

According to some embodiments of the invention the exposure to the heatcomprises heating to a temperature which is below the HDT of the firstmodeling formulation.

According to some embodiments of the invention the temperature is atleast 10° C. below the HDT of the first formulation.

According to some embodiments of the invention the temperature is abovean HDT of the second modeling formulation.

According to some embodiments of the invention the exposure to the heatcomprises heating to a temperature of at least 40° C. According to someembodiments of the invention the heating is to a temperature that rangesfrom about 40° C. to about 60° C.

According to some embodiments of the invention the exposure to the heatis effected by heat conduction. According to some embodiments of theinvention the exposure to the heat is effected by radiation. Accordingto some embodiments of the invention the exposure to the heat iseffected by heat convection.

According to some embodiments of the invention the curing energycomprises UV irradiation. According to some embodiments of the inventioneach of the curable materials in the first and the second formulationsis a UV curable material.

According to some embodiments of the invention the method comprises,subsequent to the exposing, heating the object.

According to some embodiments of the invention the heating is at atemperature of at least 120° C., and for a time period of at least 1hour.

According to some embodiments of the invention the first and the secondmodeling formulations, and a mode of the dispensing, are selected suchthat the object is characterized by HDT of at least 100° C., or at least130° C., or at least 140° C.

According to some embodiments of the invention the first and the secondmodeling formulations and a mode of the dispensing, are selected suchthat the object is characterized by Izod notch impact resistance of atleast 100 J/m.

According to some embodiments of the invention the object featurescurling of less than 4 mm, or less than 3 mm.

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise solid freeform fabrication of athree-dimensional object. The method comprises, for each of at least afew of the layers: dispensing at least a first modeling formulation anda second modeling formulation in a voxelated manner to form in the layera region in which voxels of the first modeling formulation areinterlaced with voxels and the second modeling formulation; and exposingthe layers to curing energy. Preferably, the first modeling formulationis characterized as providing, upon exposure to the curing energy, acured material featuring heat deflection temperature (HDT) of at least90° C., and the second modeling formulation is characterized asproviding, upon exposure to the curing energy, a cured materialfeaturing Izod impact resistance (IR) value of at least 45 J/m.Preferably, a ratio between elastic moduli of the first and the secondmodeling formulations, when hardened, is from less than 3; and/orwherein the first and the second modeling formulations are as delineatedabove and optionally as further exemplified below.

According to some embodiments of the invention the region is a coreregion within the layer, and the method comprises dispensing one of thefirst and the second modeling formulations but not the other one of thefirst and the second modeling formulations to form in the layer at leastone envelope region at least partially surrounding the core region.

According to some embodiments of the invention the first modelingformation is in an amount higher than 25% by weight of the total weightof the region.

According to some embodiments of the invention the first and the secondformulations and an amount of the first formulation in the region areselected such that the region, when hardened, is characterized by HDT ofat least 60° C.

According to some embodiments of the invention the invention the methodcomprises, during the dispensing, subjecting the layer to heat.

According to some embodiments of the invention the subjecting the layerto heat is as delineated above and optionally as further exemplifiedbelow.

According to some embodiments of the invention the method comprisesdispensing a plurality of layers to form a pedestal, prior to adispensing of a bottommost layer of the object.

According to some embodiments of the invention the pedestal has acore-shell structure.

According to some embodiments of the invention the dispensing theplurality of layers to form the pedestal, comprises, for each of atleast a few layers of the pedestal, dispensing a support formulation toform a core region in the layer, and a combination of a supportformulation and a modeling formulation to form in the According to someembodiments of the invention a stack of the envelope regions forms alayered shell and a stack of the core regions forms a layered core.

According to some embodiments of the invention the layered shellcomprises spaced pillars of the modeling formulation and supportformulation filling the spaces between the pillars.

According to some embodiments of the invention the layered corecomprises the support formulation and is devoid of any modelingformulation.

According to an aspect of some embodiments of the present inventionthere is provided a kit comprising at least two modeling materialformulations for fabricating a three-dimensional object. The kitcomprises at least a first modeling formulation and a second modelingformulation. The first modeling formulation preferably comprises: atleast one curable acrylic monomer characterized as providing, uponexposure to the curing energy, a cured material featuring Tg of at least85° C.; at least one curable methacrylic monomer characterized asproviding, upon exposure to the curing energy, a cured materialfeaturing Tg of at least 150° C.; at least one curable (meth)acrylicoligomer, characterized as providing, upon exposure to the curingenergy, a cured material featuring Tg of at least 50° C.; andoptionally, at least one curable (meth)acrylic monomer characterized asproviding, upon exposure to the curing energy, a cured materialfeaturing Tg lower than 0° C., wherein a concentration of the curablemethacrylic monomer is at least 35% by weight of the total weight of thefirst modeling formulation. The second modeling formulation preferablycomprises: at least one curable (meth)acrylic monomer characterized asproviding, upon exposure to the curing energy, a cured materialfeaturing Tg of at least70° C.; at least one curable (meth)acrylicoligomer characterized as providing, upon exposure to the curing energy,a cured material featuring Tg of at least 10° C.; and at least onecurable (meth)acrylic monomer which comprises at least 5 ethoxylatedgroups and is characterized by a viscosity at room temperature lowerthan 1000 centipoises; and a molecular weight of at least 500 grams/mol,wherein a concentration of the ethoxylated curable material is at least5 weight percents of the total weight of the second modelingformulation.

According to some embodiments of the invention the first and secondmodeling formulations are packaged separately within the kit.

According to some embodiments of the invention the kit comprises atleast one initiator for initiating curing of the first and/or the secondformulation.

According to an aspect of some embodiments of the present inventionthere is provided a method of fabricating a three-dimensional object.The method comprises: within a chamber of solid freeform fabricationapparatus, sequentially forming a plurality of layers in a configuredpattern corresponding to the shape of the object, wherein, for each ofat least a few of the layers, the formation of the layer comprises:dispensing at least a first modeling formulation and a second modelingformulation to form a core region using both the first and the secondmodeling formulations, and at least one envelope region at leastpartially surrounding the core region using one of the first and thesecond modeling formulations but not the other one of the first and thesecond modeling formulations. The method preferably comprises exposingthe layer to non-thermal curing energy, and heating the chamber to atemperature of at least 40° C.

According to an aspect of some embodiments of the present inventionthere is provided a three-dimensional printing system. The systemcomprises: a plurality of inkjet printing heads each having a circuitcontrollably dispensing a different type of modeling material; a trayfor receiving modeling materials dispensed by the inkjet printing headsa curing device configured for applying curing energy; a thermal screenfor thermally separating the circuits from a space between the inkjetprinting heads and the tray; and a heating system for heating the space.

According to some embodiments of the invention the heating systemcomprises a source of thermal radiation positioned in the space todeliver heat to the dispensed modeling material by radiation.

According to some embodiments of the invention the heating systemcomprises a blower positioned outside the space for delivering heat tothe dispensed modeling material by convection.

According to some embodiments of the invention the heating systemcomprises a tray heater in thermal contact with a back side of the trayfor delivering heat to the dispensed modeling material by heatconduction.

According to some embodiments of the invention the thermal screen isfoldable and collapsible, and is positioned to simultaneously fold atone side of the inkjet printing heads and expand at an opposite side ofinkjet printing heads during a motion of the inkjet printing heads.

According to an aspect of some embodiments of the present inventionthere is provided a three-dimensional object obtained in layerwise solidfreeform fabrication. The object comprises at least one core region andat least one envelope region at least partially surrounding the coreregion, the object being characterized by: a heat deflection temperature(HDT) of at least 100° C.; an Izod impact resistance (IR) value of atleast 100 J/m; and a curling of less than 4 mm, or less than 3 mm.

According to an aspect of some embodiments of the present inventionthere is provided a method of layerwise fabrication of athree-dimensional object. The method comprises: dispensing a pluralityof pedestal layers to form a layered pedestal having a core-shellstructure; and dispensing a plurality of object layers to form theobject on top of the pedestal.

According to some embodiments of the invention the dispensing of theplurality of pedestal layers, comprises, for at least a few pedestallayers, dispensing a support formulation to form a core region in thelayer, and a combination of a support formulation and a modelingformulation to form in the layer an envelope region at least partiallysurrounding the core region, wherein a stack of the envelope regionsforms a layered shell and a stack of the core regions forms a layeredcore.

According to some embodiments of the invention the core-shell structurecomprises a layered core made of a first support formulation and beingdevoid of any modeling formulation, and a layered shell formed of acombination of a modeling formulation and a second support formulation.

According to some embodiments of the invention the core-shell structurecomprises a layered shell having spaced pillars made of a modelingformulation, and a support formulation filling the spaces between thepillars.

According to some embodiments of the invention the dispensing theplurality of object layers comprises, for at least a few object layers,dispensing a first modeling formulation and a second modelingformulation to form a core region using both the first and the secondmodeling formulations, and at least one envelope region at leastpartially surrounding the core region using one of the first and thesecond modeling formulations but not the other one of the first and thesecond modeling formulations.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIGS. 1A-1G show results of computer simulations performed in accordancewith some embodiments of the present invention to analyze stressdistribution resulting from a crack in a modeling formulation;

FIGS. 2A-2B show the effect of various concentrations of exemplaryformulations according to some embodiments of the present invention (aformulation referred to as RF4w in FIG. 2A and a formulation referred toas RF71 in FIG. 2B) in a core on the HDT of the various printed objects;

FIGS. 3A-3B show the effect of various concentrations of exemplaryformulations according to some embodiments of the present invention, aformulation referred to as RF4w and a formulation referred to as RF71(FIG. 3A) in a core on the Impact resistance of the various printedobjects, compared to an effect of various concentrations of a previouslydescribed formulation referred to herein as RF535 (FIG. 3B);

FIG. 4 is a graph of a temperature ensuring curling of less than 3 mm asa function of the final HDT of the printed object, for four differentformulation or combination of formulations;

FIGS. 5A-5D are schematic illustrations of an additive manufacturingsystem according to some embodiments of the invention;

FIGS. 6A-6C are schematic illustrations of printing heads according tosome embodiments of the present invention;

FIG. 7 is a schematic illustration of an additive manufacturing systemin embodiments of the invention in which the system comprises a thermalscreen;

FIG. 8 is a graph showing a typical linear dependence of a temperatureon voltage applied to an infrared source;

FIGS. 9A-9F are schematic illustrations of shelled structures, accordingto some embodiments of the present invention;

FIGS. 10A-10B are schematic illustrations of an object formed on apedestal, according to some embodiments of the present invention;

FIG. 11 is a schematic illustration of a shelled structure having partsthat are devoid of a core region, according to some embodiments of thepresent invention;

FIGS. 12A-12B present the effect of various concentrations of anexemplary modeling material formulations according to some embodimentsof the present invention (referred to as RF4w) in the core of afabricated object on the Loss modulus (FIG. 12A) and on the storagemodulus (FIG. 12B) of the object at various temperatures;

FIGS. 13A-13B present the effect of various concentrations of anexemplary modeling material formulations according to some embodimentsof the present invention (referred to as RF71) in the core of afabricated object on the Loss modulus (FIG. 13A) and on the storagemodulus (FIG. 13B) of the object at various temperatures;

FIGS. 14A-14B present the effect of exemplary modeling materialformulations according to some embodiments of the present invention(referred to as RF4w and RF71), and of RF535, when used at a 50/50weight ratio (FIG. 14A) and at a 25/75 weight ratio (FIG. 14B) with asecond modeling material formulation according to some embodiments ofthe present invention, on the storage modulus of the object at varioustemperatures; and

FIGS. 15A-15B present the effect of exemplary modeling materialformulations according to some embodiments of the present invention(referred to as RF4w and RF71), and of RF535, when used at a 25/75weight ratio (FIG. 15A) and at a 50/50 weight ratio (FIG. 15B) with asecond modeling material formulation according to some embodiments ofthe present invention, in a dynamic mechanical analysis of an objectfabricated according to some embodiments of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to AdditiveManufacturing (AM) of an object, and, more particularly, but notexclusively, to formulations, methods and systems for additivemanufacturing of an object which exhibits desirable mechanicalproperties, for example, a desirable Heat Deflection Temperature (HDT)without compromising other properties.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

The method and system of the present embodiments manufacturethree-dimensional objects based on computer object data in a layerwisemanner by forming a plurality of layers in a configured patterncorresponding to the shape of the objects. The computer object data canbe in any known format, including, without limitation, a StandardTessellation Language (STL) or a StereoLithography Contour (SLC) format,Virtual Reality Modeling Language (VRML), Additive Manufacturing File(AMF) format, Drawing Exchange Format (DXF), Polygon File Format (PLY)or any other format suitable for Computer-Aided Design (CAD).

The term “object” as used herein refers to a whole object or a partthereof.

Each layer is formed by additive manufacturing apparatus which scans atwo-dimensional surface and patterns it. While scanning, the apparatusvisits a plurality of target locations on the two-dimensional layer orsurface, and decides, for each target location or a group of targetlocations, whether or not the target location or group of targetlocations is to be occupied by building material, and which type ofbuilding material is to be delivered thereto. The decision is madeaccording to a computer image of the surface.

In preferred embodiments of the present invention the AM comprisesthree-dimensional printing, more preferably three-dimensional inkjetprinting. In these embodiments a building material is dispensed from adispensing head having a circuit for controllably dispensing buildingmaterial in layers on a supporting structure. Typically, each dispensinghead optionally and preferably has a set of nozzles to deposit buildingmaterial in layers on a supporting structure. The AM apparatus thusdispenses building material in target locations which are to be occupiedand leaves other target locations void. The apparatus typically includesa plurality of dispensing heads, each of which can be configured todispense a different building material. Thus, different target locationscan be occupied by different building materials. The types of buildingmaterials can be categorized into two major categories: modelingmaterial and support material. The support material serves as asupporting matrix or construction for supporting the object or objectparts during the fabrication process and/or other purposes, e.g.,providing hollow or porous objects. Support constructions mayadditionally include modeling material elements, e.g. for furthersupport strength.

The modeling material is generally a composition which is formulated foruse in additive manufacturing and which is able to form athree-dimensional object on its own, i.e., without having to be mixed orcombined with any other substance.

The final three-dimensional object is made of the modeling material or acombination of modeling materials or modeling and support materials ormodification thereof (e.g., following curing). All these operations arewell-known to those skilled in the art of solid freeform fabrication.

In some exemplary embodiments of the invention an object is manufacturedby dispensing two or more different modeling materials, each materialfrom a different dispensing head of the AM. The materials are optionallyand preferably deposited in layers during the same pass of thedispensing heads. The materials and combination of materials within thelayer are selected according to the desired properties of the object.

A representative and non-limiting example of a system 110 suitable forAM of an object 112 according to some embodiments of the presentinvention is illustrated in FIG. 5A. System 110 comprises an additivemanufacturing apparatus 114 having a dispensing unit 16 which comprisesa plurality of dispensing heads. Each head preferably comprises an arrayof one or more nozzles 122, as illustrated in FIGS. 6A-6C describedbelow, through which a liquid building material 124 is dispensed.

Preferably, but not obligatorily, apparatus 114 is a three-dimensionalprinting apparatus, in which case the dispensing heads are printingheads, and the building material is dispensed via inkjet technology.This need not necessarily be the case, since, for some applications, itmay not be necessary for the additive manufacturing apparatus to employthree-dimensional printing techniques. Representative examples ofadditive manufacturing apparatus contemplated according to variousexemplary embodiments of the present invention include, withoutlimitation, fused deposition modeling apparatus and fused materialdeposition apparatus.

Each dispensing head is optionally and preferably fed via a buildingmaterial reservoir which may optionally include a temperature controlunit (e.g., a temperature sensor and/or a heating device), and amaterial level sensor. To dispense the building material, a voltagesignal is applied to the dispensing heads to selectively depositdroplets of material via the dispensing head nozzles, for example, as inpiezoelectric inkjet printing technology. The dispensing rate of eachhead depends on the number of nozzles, the type of nozzles and theapplied voltage signal rate (frequency). Such dispensing heads are knownto those skilled in the art of solid freeform fabrication.

Preferably, but not obligatorily, the overall number of dispensingnozzles or nozzle arrays is selected such that half of the dispensingnozzles are designated to dispense support material and half of thedispensing nozzles are designated to dispense modeling material, i.e.the number of nozzles jetting modeling materials is the same as thenumber of nozzles jetting support material. In the representativeexample of FIG. 5A, four dispensing heads 16 a, 16 b, 16 c and 16 d areillustrated. Each of heads 16 a, 16 b, 16 c and 16 d has a nozzle array.In this Example, heads 16 a and 16 b can be designated for modelingmaterial/s and heads 16 c and 16 d can be designated for supportmaterial. Thus, head 16 a can dispense a first modeling material, head16 b can dispense a second modeling material and heads 16 c and 16 d canboth dispense support material. In an alternative embodiment, heads 16 cand 16 d, for example, may be combined in a single head having twonozzle arrays for depositing support material.

Yet it is to be understood that it is not intended to limit the scope ofthe present invention and that the number of modeling materialdepositing heads (modeling heads) and the number of support materialdepositing heads (support heads) may differ. Generally, the number ofmodeling heads, the number of support heads and the number of nozzles ineach respective head or head array are selected such as to provide apredetermined ratio, a, between the maximal dispensing rate of thesupport material and the maximal dispensing rate of modeling material.The value of the predetermined ratio, a, is preferably selected toensure that in each formed layer, the height of modeling material equalsthe height of support material. Typical values for a are from about 0.6to about 1.5.

As used herein throughout, the term “about” refers to ±10% or to ±5%.

For example, for a=1, the overall dispensing rate of support material isgenerally the same as the overall dispensing rate of the modelingmaterial when all modeling heads and support heads operate.

In a preferred embodiment, there are M modeling heads each having marrays of p nozzles, and S support heads each having s arrays of qnozzles such that M×m×p=S×s×q. Each of the M×m modeling arrays and S×ssupport arrays can be manufactured as a separate physical unit, whichcan be assembled and disassembled from the group of arrays. In thisembodiment, each such array optionally and preferably comprises atemperature control unit and a material level sensor of its own, andreceives an individually controlled voltage for its operation.

Apparatus 114 can further comprise a hardening device 324 which caninclude any device configured to emit light, heat or the like that maycause the deposited material to hardened. For example, hardening device324 can comprise one or more radiation sources, which can be, forexample, an ultraviolet or visible or infrared lamp, or other sources ofelectromagnetic radiation, or electron beam source, depending on themodeling material being used. In some embodiments of the presentinvention, hardening device 324 serves for curing or solidifying themodeling material.

The dispensing head and radiation source are preferably mounted in aframe or block 128 which is preferably operative to reciprocally moveover a tray 360, which serves as the working surface. In someembodiments of the present invention the radiation sources are mountedin the block such that they follow in the wake of the dispensing headsto at least partially cure or solidify the materials just dispensed bythe dispensing heads. Tray 360 is positioned horizontally. According tothe common conventions an X-Y-Z Cartesian coordinate system is selectedsuch that the X-Y plane is parallel to tray 360. Tray 360 is preferablyconfigured to move vertically (along the Z direction), typicallydownward. In various exemplary embodiments of the invention, apparatus114 further comprises one or more leveling devices 132, e.g. a roller326. Leveling device 326 serves to straighten, level and/or establish athickness of the newly formed layer prior to the formation of thesuccessive layer thereon. Leveling device 326 preferably comprises awaste collection device 136 for collecting the excess material generatedduring leveling. Waste collection device 136 may comprise any mechanismthat delivers the material to a waste tank or waste cartridge.

In use, the dispensing heads of unit 16 move in a scanning direction,which is referred to herein as the X direction, and selectively dispensebuilding material in a predetermined configuration in the course oftheir passage over tray 360. The building material typically comprisesone or more types of support material and one or more types of modelingmaterial. The passage of the dispensing heads of unit 16 is followed bythe curing of the modeling material(s) by radiation source 126. In thereverse passage of the heads, back to their starting point for the layerjust deposited, an additional dispensing of building material may becarried out, according to predetermined configuration. In the forwardand/or reverse passages of the dispensing heads, the layer thus formedmay be straightened by leveling device 326, which preferably follows thepath of the dispensing heads in their forward and/or reverse movement.Once the dispensing heads return to their starting point along the Xdirection, they may move to another position along an indexingdirection, referred to herein as the Y direction, and continue to buildthe same layer by reciprocal movement along the X direction.Alternately, the dispensing heads may move in the Y direction betweenforward and reverse movements or after more than one forward-reversemovement. The series of scans performed by the dispensing heads tocomplete a single layer is referred to herein as a single scan cycle.

Once the layer is completed, tray 360 is lowered in the Z direction to apredetermined Z level, according to the desired thickness of the layersubsequently to be printed. The procedure is repeated to formthree-dimensional object 112 in a layerwise manner.

In another embodiment, tray 360 may be displaced in the Z directionbetween forward and reverse passages of the dispensing head of unit 16,within the layer. Such Z displacement is carried out in order to causecontact of the leveling device with the surface in one direction andprevent contact in the other direction.

System 110 optionally and preferably comprises a building materialsupply system 330 which comprises the building material containers orcartridges and supplies a plurality of building materials to fabricationapparatus 114.

A control unit 340 controls fabrication apparatus 114 and optionally andpreferably also supply system 330. Control unit 340 typically includesan electronic circuit configured to perform the controlling operations.Control unit 340 preferably communicates with a data processor 154 whichtransmits digital data pertaining to fabrication instructions based oncomputer object data, e.g., a CAD configuration represented on acomputer readable medium in a form of a Standard Tessellation Language(STL) format or the like. Typically, control unit 340 controls thevoltage applied to each dispensing head or nozzle array and thetemperature of the building material in the respective printing head.

Once the manufacturing data is loaded to control unit 340 it can operatewithout user intervention. In some embodiments, control unit 340receives additional input from the operator, e.g., using data processor154 or using a user interface 116 communicating with unit 340. Userinterface 116 can be of any type known in the art, such as, but notlimited to, a keyboard, a touch screen and the like. For example,control unit 340 can receive, as additional input, one or more buildingmaterial types and/or attributes, such as, but not limited to, color,characteristic distortion and/or transition temperature, viscosity,electrical property, magnetic property. Other attributes and groups ofattributes are also contemplated.

Another representative and non-limiting example of a system 10 suitablefor AM of an object according to some embodiments of the presentinvention is illustrated in FIGS. 5B-5D. FIGS. 5B-5D illustrate a topview (FIG. 5B), a side view (FIG. 5C) and an isometric view (FIG. 5D) ofsystem 10.

In the present embodiments, system 10 comprises a tray 12 and aplurality of inkjet printing heads 16, each having a plurality ofseparated nozzles. Tray 12 can have a shape of a disk or it can beannular. Non-round shapes are also contemplated, provided they can berotated about a vertical axis.

Tray 12 and heads 16 are optionally and preferably mounted such as toallow a relative rotary motion between tray 12 and heads 16. This can beachieved by (i) configuring tray 12 to rotate about a vertical axis 14relative to heads 16, (ii) configuring heads 16 to rotate about verticalaxis 14 relative to tray 12, or (iii) configuring both tray 12 and heads16 to rotate about vertical axis 14 but at different rotation velocities(e.g., rotation at opposite direction). While the embodiments below aredescribed with a particular emphasis to configuration (i) wherein thetray is a rotary tray that is configured to rotate about vertical axis14 relative to heads 16, it is to be understood that the presentapplication contemplates also configurations (ii) and (iii). Any one ofthe embodiments described herein can be adjusted to be applicable to anyof configurations (ii) and (iii), and one of ordinary skills in the art,provided with the details described herein, would know how to make suchadjustment.

In the following description, a direction parallel to tray 12 andpointing outwardly from axis 14 is referred to as the radial directionr, a direction parallel to tray 12 and perpendicular to the radialdirection r is referred to herein as the azimuthal direction φ, and adirection perpendicular to tray 12 is referred to herein is the verticaldirection z.

The term “radial position,” as used herein, refers to a position on orabove tray 12 at a specific distance from axis 14. When the term is usedin connection to a printing head, the term refers to a position of thehead which is at specific distance from axis 14. When the term is usedin connection to a point on tray 12, the term corresponds to any pointthat belongs to a locus of points that is a circle whose radius is thespecific distance from axis 14 and whose center is at axis 14.

The term “azimuthal position,” as used herein, refers to a position onor above tray 12 at a specific azimuthal angle relative to apredetermined reference point. Thus, radial position refers to any pointthat belongs to a locus of points that is a straight line forming thespecific azimuthal angle relative to the reference point.

The term “vertical position,” as used herein, refers to a position overa plane that intersect the vertical axis 14 at a specific point.

Tray 12 serves as a supporting structure for three-dimensional printing.The working area on which one or objects are printed is typically, butnot necessarily, smaller than the total area of tray 12. In someembodiments of the present invention the working area is annular. Theworking area is shown at 26. In some embodiments of the presentinvention tray 12 rotates continuously in the same direction throughoutthe formation of object, and in some embodiments of the presentinvention tray reverses the direction of rotation at least once (e.g.,in an oscillatory manner) during the formation of the object. Tray 12 isoptionally and preferably removable. Removing tray 12 can be formaintenance of system 10, or, if desired, for replacing the tray beforeprinting a new object. In some embodiments of the present inventionsystem 10 is provided with one or more different replacement trays(e.g., a kit of replacement trays), wherein two or more trays aredesignated for different types of objects (e.g., different weights)different operation modes (e.g., different rotation speeds), etc. Thereplacement of tray 12 can be manual or automatic, as desired. Whenautomatic replacement is employed, system 10 comprises a trayreplacement device 36 configured for removing tray 12 from its positionbelow heads 16 and replacing it by a replacement tray (not shown). Inthe representative illustration of FIG. 5B tray replacement device 36 isillustrated as a drive 38 with a movable arm 40 configured to pull tray12, but other types of tray replacement devices are also contemplated.

Exemplified embodiments for the printing head 16 are illustrated inFIGS. 6A-6C. These embodiments can be employed for any of the AM systemsdescribed above, including, without limitation, system 110 and system10.

FIGS. 6A-6B illustrate a printing head 16 with one (FIG. 6A) and two(FIG. 6B) nozzle arrays 22. The nozzles in the array are preferablyaligned linearly, along a straight line. In embodiments in which aparticular printing head has two or more linear nozzle arrays, thenozzle arrays are optionally and preferably can be parallel to eachother.

When a system similar to system 110 is employed, all printing heads 16are optionally and preferably oriented along the indexing direction withtheir positions along the scanning direction being offset to oneanother.

When a system similar to system 10 is employed, all printing heads 16are optionally and preferably oriented radially (parallel to the radialdirection) with their azimuthal positions being offset to one another.Thus, in these embodiments, the nozzle arrays of different printingheads are not parallel to each other but are rather at an angle to eachother, which angle being approximately equal to the azimuthal offsetbetween the respective heads. For example, one head can be orientedradially and positioned at azimuthal position φ₁, and another head canbe oriented radially and positioned at azimuthal position φ₂. In thisexample, the azimuthal offset between the two heads is φ₁-φ₂, and theangle between the linear nozzle arrays of the two heads is also φ₁-φ₂.

In some embodiments, two or more printing heads can be assembled to ablock of printing heads, in which case the printing heads of the blockare typically parallel to each other. A block including several inkjetprinting heads 16 a, 16 b, 16 c is illustrated in FIG. 6C.

In some embodiments, system 10 comprises a support structure 30positioned below heads 16 such that tray 12 is between support structure30 and heads 16. Support structure 30 may serve for preventing orreducing vibrations of tray 12 that may occur while inkjet printingheads 16 operate. In configurations in which printing heads 16 rotateabout axis 14, support structure 30 preferably also rotates such thatsupport structure 30 is always directly below heads 16 (with tray 12between heads 16 and tray 12).

Tray 12 and/or printing heads 16 is optionally and preferably configuredto move along the vertical direction z, parallel to vertical axis 14 soas to vary the vertical distance between tray 12 and printing heads 16.In configurations in which the vertical distance is varied by movingtray 12 along the vertical direction, support structure 30 preferablyalso moves vertically together with tray 12. In configurations in whichthe vertical distance is varied by heads 16 along the verticaldirection, while maintaining the vertical position of tray 12 fixed,support structure 30 is also maintained at a fixed vertical position.

The vertical motion can be established by a vertical drive 28. Once alayer is completed, the vertical distance between tray 12 and heads 16can be increased (e.g., tray 12 is lowered relative to heads 16) by apredetermined vertical step, according to the desired thickness of thelayer subsequently to be printed. The procedure is repeated to form athree-dimensional object in a layerwise manner.

The operation of inkjet printing heads 16 and optionally and preferablyalso of one or more other components of system 10, e.g., the motion oftray 12, are controlled by a controller 20. The controller can has anelectronic circuit and a non-volatile memory medium readable by thecircuit, wherein the memory medium stores program instructions which,when read by the circuit, cause the circuit to perform controloperations as further detailed below.

Controller 20 can also communicate with a host computer 24 whichtransmits digital data pertaining to fabrication instructions based oncomputer object data, e.g., in a form of a Standard TessellationLanguage (STL) or a StereoLithography Contour (SLC) format, VirtualReality Modeling Language (VRML), Additive Manufacturing File (AMF)format, Drawing Exchange Format (DXF), Polygon File Format (PLY) or anyother format suitable for Computer-Aided Design (CAD). The object dataformats are typically structured according to a Cartesian system ofcoordinates. In these cases, computer 24 preferably executes a procedurefor transforming the coordinates of each slice in the computer objectdata from a Cartesian system of coordinates into a polar system ofcoordinates. Computer 24 optionally and preferably transmits thefabrication instructions in terms of the transformed system ofcoordinates. Alternatively, computer 24 can transmit the fabricationinstructions in terms of the original system of coordinates as providedby the computer object data, in which case the transformation ofcoordinates is executed by the circuit of controller 20.

The transformation of coordinates allows three-dimensional printing overa rotating tray. In conventional three-dimensional printing, theprinting heads reciprocally move above a stationary tray along straightlines. In such conventional systems, the printing resolution is the sameat any point over the tray, provided the dispensing rates of the headsare uniform. Unlike conventional three-dimensional printing, not all thenozzles of the head points cover the same distance over tray 12 duringat the same time. The transformation of coordinates is optionally andpreferably executed so as to ensure equal amounts of excess material atdifferent radial positions.

Typically, controller 20 controls the voltage applied to the respectivecomponent of the system 10 based on the fabrication instructions andbased on the stored program instructions as described below.

Generally, controller 20 controls printing heads 16 to dispense, duringthe rotation of tray 12, droplets of building material in layers, suchas to print a three-dimensional object on tray 12.

System 10 optionally and preferably comprises one or more radiationsources 18, which can be, for example, an ultraviolet or visible orinfrared lamp, or other sources of electromagnetic radiation, orelectron beam source, depending on the modeling material being used.Radiation source can include any type of radiation emitting device,including, without limitation, light emitting diode (LED), digital lightprocessing (DLP) system, resistive lamp and the like. Radiation source18 serves for curing or solidifying the modeling material. In variousexemplary embodiments of the invention the operation of radiation source18 is controlled by controller 20 which may activate and deactivateradiation source 18 and may optionally also control the amount ofradiation generated by radiation source 18.

In some embodiments of the invention, system 10 further comprises one ormore leveling devices 32 which can be manufactured as a roller or ablade. Leveling device 32 serves to straighten the newly formed layerprior to the formation of the successive layer thereon. In someembodiments, leveling device 32 has the shape of a conical rollerpositioned such that its symmetry axis 34 is tilted relative to thesurface of tray 12 and its surface is parallel to the surface of thetray. This embodiment is illustrated in the side view of system 10 (FIG.6C).

The conical roller can have the shape of a cone or a conical frustum.

The opening angle of the conical roller is preferably selected such thatthere is a constant ratio between the radius of the cone at any locationalong its axis 34 and the distance between that location and axis 14.This embodiment allows roller 32 to efficiently level the layers, sincewhile the roller rotates, any point p on the surface of the roller has alinear velocity which is proportional (e.g., the same) to the linearvelocity of the tray at a point vertically beneath point p. In someembodiments, the roller has a shape of a conical frustum having a heighth, a radius R₁ at its closest distance from axis 14, and a radius R₂ atits farthest distance from axis 14, wherein the parameters h, R₁ and R₂satisfy the relation R₁R₂=(R−h)/h and wherein R is the farthest distanceof the roller from axis 14 (for example, R can be the radius of tray12).

The operation of leveling device 32 is optionally and preferablycontrolled by controller 20 which may activate and deactivate levelingdevice 32 and may optionally also control its position along a verticaldirection (parallel to axis 14) and/or a radial direction (parallel totray 12 and pointing toward or away from axis 14.

In some embodiments of the present invention printing heads 16 areconfigured to reciprocally move relative to tray along the radialdirection r. These embodiments are useful when the lengths of the nozzlearrays 22 of heads 16 are shorter than the width along the radialdirection of the working area 26 on tray 12. The motion of heads 16along the radial direction is optionally and preferably controlled bycontroller 20.

Some embodiments contemplate the fabrication of an object by dispensingdifferent materials from different dispensing heads. These embodimentsprovide, inter alia, the ability to select materials from a given numberof materials and define desired combinations of the selected materialsand their properties. According to the present embodiments, the spatiallocations of the deposition of each material with the layer is defined,either to effect occupation of different three-dimensional spatiallocations by different materials, or to effect occupation ofsubstantially the same three-dimensional location or adjacentthree-dimensional locations by two or more different materials so as toallow post deposition spatial combination of the materials within thelayer, thereby to form a composite material at the respective locationor locations.

Any post deposition combination or mix of modeling materials iscontemplated. For example, once a certain material is dispensed it maypreserve its original properties. However, when it is dispensedsimultaneously with another modeling material or other dispensedmaterials which are dispensed at the same or nearby locations, acomposite material having a different property or properties to thedispensed materials is formed.

The present embodiments thus enable the deposition of a broad range ofmaterial combinations, and the fabrication of an object which mayconsist of multiple different combinations of materials, in differentparts of the object, according to the properties desired to characterizeeach part of the object.

Further details on the principles and operations of an AM systemsuitable for the present embodiments are found in U.S. PublishedApplication No. 20100191360, and International Publication No.WO2016/009426, the contents of which are hereby incorporated byreference.

The systems of the present embodiments (system 10 and system 110) areoptionally and preferably supplemented with a thermal screen forthermally separating the circuits of the dispensing or printing headsfrom the space between the heads and the tray. A representativeschematic illustration of this embodiment is shown in FIG. 7. FIG. 7illustrates the system for the case of linear relative motion betweenthe tray and the heads, but the ordinarily skilled person, provided withthe details described herein, would know how to adjust the drawing forthe case in which the relative motion is rotary (for example, byreplacing tray 360 of with tray 12, and printing block 128 by at leastone of printing head 16, radiation source 18 and leveling device 32).For clarity of presentation, several features of the systems such as thesupply apparatus, the user interface and the controller have beenomitted from FIG. 7.

Shown in FIG. 7 is a printing chamber 700 having therein the tray 360and the printing block 128. Block 128 represents all the elements thatare used for dispensing and hardening the building materials, including,without limitation, the printing heads, leveling devices and hardeningdevices. The electronic circuits of block 128 (e.g., the electroniccircuit of the printing heads, leveling devices and/or hardeningdevices) are located at the upper part of block 128 and are collectivelyshown at 704. The lower part of block 128 can include the nozzles 122 ofthe printing heads, the mechanical parts of leveling device 326 and theoutput of the hardening device 324).

A thermal screen 702 separates between the upper part 700 a and lowerpart 700 b of chamber 700, such that the upper part of block 128,including electronic circuits 704, is above thermal screen 702, and thelower part of block 128 is below thermal screen 702. This ensures thatthe electronic circuits 704 are thermally separated from the componentsof block 128 that dispense or otherwise interact (mechanically or by wayof radiation) with the building material. This embodiment is useful whenthe system is used to dispense the first and/or second modelingformulations described herein, particularly when heat is applied to theformulations, for example, for reducing the curling effect.

Thermal screen 72 is optionally and preferably collapsible andexpandable, and is positioned to simultaneously fold at one side ofblock 128 and expand at an opposite side of block 128 during the motionof block 128. For example, screen 702 can be structured as an accordionfolding screen or as a telescopic screen, e.g., a concentric coupling ofa series of nested and axially interlocking hollow structures ofgradually reducing dimensions. Screen 702 can be made of, or coated by,a thermally reflective material.

In some embodiments of the present invention system 110 (or system 10)comprises a heating system 706 that heats the lower part 700 b ofchamber 700, particularly the space between the printing head and thetray. Heating system 706 can be embodied in more than one way. In someembodiments of the present invention, heating system 706 comprises atray heater 708 in thermal contact with a back side of tray 360 fordelivering heat to the modeling material that is dispensed on the frontside of the tray by heat conduction.

Tray heater 708 can comprises one or more heating panels havingresistance filament. When tray heater 708 is employed, tray 360 is madeof a heat conductive material, such as a metal, e.g., aluminum.Typically, but not necessarily, the resistance filament can be coated byor embedded in an encapsulation, such as, but not limited to, a siliconencapsulation or the like. The heating panel(s) are preferably disposedso as to cover the entire back side of the working area of tray 360. Thetemperature of heater 708 can be controlled by a temperature controlcircuit 714, such as, but not limited to, aproportional-integral-derivative (PID) controller. Temperature controlcircuit 714 can receive temperature data from a temperature sensor 716,such as, but not limited to, a thermocouple, positioned in contact withheater 708 and control the voltage on the resistance filamentresponsively to the received temperature data and to control signalsreceived from the main controller (not shown, see 152 in FIG. 5A and 20in FIG. 5B).

Typical operational parameters of tray heater 708 are, withoutlimitation, temperature range of 40-100° C., maximal flux about 1-2w/cm², e.g., about 1.5 w/cm², maximal applied voltage about 360-400volts, e.g., about 380 volts, or from about 150 volts to about 230volts.

In some embodiments of the present invention heating system 706comprises a radiation source 718 that delivers heat to said dispensedmodeling material by radiation (e.g., infrared radiation). Radiativeheat is optionally and preferably applied from the top side of the trayso as to allow heating dispensed material that is farther from the tray.For example, source 718 can be mounted on block 128 (e.g., alongsidedevice 324), so as to allow it to move reciprocally over the tray. Theradiation source 718 can be controlled by a temperature control circuit720, such as, but not limited to, a (PID) controller, which receivestemperature data from a temperature sensor (not shown) that is mountedon the source 718, and provides voltage pulses to the sourceresponsively to the received temperature data and to control signalsreceived from the main controller (not shown, see 152 in FIG. 5A and 20in FIG. 5B). Alternatively, an open loop control can be employed, inwhich case a constant voltage level is applied to the source withoutdynamically controlling the voltage based on temperature data.

Typical operational parameters of infrared source 718 are, withoutlimitation, temp range of 40-900° C., wavelength range 2-10 μm, maximalflux 6-7 w/cm², e.g., about 6.4 w/cm², voltage 150-400 volts, e.g.,about 180 volts. FIG. 8 is a graph showing a typical linear dependenceof the temperature inside infrared source 718 as a function of thevoltage applied to source 718.

In some embodiments of the present invention, heating system 706comprises a chamber heater 712 for delivering heat to the modelingmaterial that is dispensed on the front side of the tray by heatconduction. In some embodiments of the present invention, heating system706 comprises a blower and/or fan 710 positioned outside the spacebetween the block 128 and the tray 360 (e.g., below the tray) fordelivering heat to the dispensed modeling material by convection. Heatconvection (e.g., by air) is generally shown by block arrows. Use ofchamber heater 712 optionally and preferably in combination with blowerand/or fan 710 is advantageous because it allows heating also the sidewalls and the top of the printed object. Preferably, the chamber heater712 is activated before (e.g., 10-60 minutes before) the dispensing ofbuilding material is initiated.

The chamber heater 712 and/or blower and/or fan 710 is or can becontrolled by a temperature control circuit 722, such as, but notlimited to, a (PID) controller, which receives temperature data from atemperature sensor 724 that is mounted in the space between block 128and tray 360, and controls the temperature of chamber heater 712 and/orthe fan speed of blower or fan 710 responsively to the receivedtemperature data and to control signals received from the maincontroller (not shown, see 152 in FIG. 5A and 20 in FIG. 5B). Themaximal temperature of heater 712 is, without limitation 600-700° C.,e.g., about 650° C., and the maximal air flow generated by blower or fan710 is, without limitation 250-350 l/min, e.g., about 300 l/min.

Preferably, heating system 706 includes two or more of the aboveelements, so as to allow system 706 to deliver heat by at least twomechanisms selected from the group consisting of heat conduction, heatconvection and radiation. In some embodiments of the present inventionthe controller (not shown, see 152 in FIG. 5A and 20 in FIG. 5B)receives from the user interface a heating mode and operates heatingsystem according to received mode. The mode can be selected from apredetermined list heating modes. For example, in one heating mode, thetray heater, infrared radiation, chamber heater and blower or fan areoperated. In another heating mode, the tray heater, infrared radiationand chamber heater are operated, but the blower or fan is not operated.In another heating mode, the tray heater and infrared radiation areoperated, but the chamber heater and blower or fan is not operated. Inanother heating mode, the tray heater, infrared radiation and blower orfan are operated, but the chamber heater is not operated. In anotherheating mode, the tray heater and chamber heater is operated, but theinfrared radiation and blower or fan are not operated. In anotherheating mode, the tray heater, chamber heater is operated and blower orfan are operated, but the infrared radiation is not operated. Typically,but not necessarily the user interface displays this list of modes tothe user and allows the user to select the desired mode.

It is recognized that some modeling materials, particularly UVpolymerizable materials, exhibit undesired deformation such as curlingduring the freeform fabrication of the object. In a search made by theinventors of the present invention for a solution to the problem ofcurling, it was found that the extent of curling is proportional to theextent of volumetric shrinkage that the material undergoes during thepolymerization process and the temperature difference between materialHDT and the system's temperature during fabrication. The presentinventors found that the curling is particularly noticeable formaterials with relatively high volumetric shrinkage and relatively highHDT (e.g., within the range of the polymerization temperature).

The present inventors have devised a layered (e.g., polymeric) object orstructure which enjoys thermo-mechanical properties which are improvedcompared to other objects fabricated via AM.

Generally, the structure according to various exemplary embodiments ofthe present invention is a shelled structure made of two or moremodeling formulations (e.g., UV polymerizable modeling formulations).The structure typically comprises a layered core which is at leastpartially coated by one or more layered shells such that at least onelayer of the core engages the same plane with a layer of at least one ofthe shells. The thickness of each shell, as measured perpendicularly tothe surface of the structure, is typically at least 10 μm. In variousexemplary embodiments of the invention, the core and the shell aredifferent from each other in their thermo-mechanical properties. This isreadily achieved by fabricating the core and shell from differentmodeling formulations or different combinations of modelingformulations. The thermo-mechanical properties of the core and shell arereferred to herein as “core thermo-mechanical properties” and “shellthermo-mechanical properties,” respectively.

A representative and non-limiting example of a structure according tosome embodiments of the present invention is shown in FIGS. 9A-9D.

FIG. 9A is a schematic illustration of a perspective view of a structure60, and FIG. 9B is a cross-sectional view of structure 60 along line A-Aof FIG. 9A. For clarity of presentation a Cartesian coordinate system isalso illustrated.

Structure 60 comprises a plurality of layers 62 stacked along the zdirection. Structure 60 is typically fabricated by an AM technique,e.g., using system 10, whereby the layers are formed in a sequentialmanner. Thus, the z direction is also referred to herein as the “builddirection” of the structure. Layers 62 are, therefore, perpendicular tothe build direction. Although structure 60 is shown as a cylinder, thisneed not necessarily be the case, since the structure of the presentembodiments can have any shape.

The shell and core of structure 60 are shown at 64 and 66, respectively.As shown, the layers of core 66 and the layers of shell 64 areco-planar. The AM technique allows the simultaneous fabrication of shell64 and core 66, whereby for a particular formed layer, the inner part ofthe layer constitutes a layer of the core, and the periphery of thelayer, or part thereof, constitutes a layer of the shell.

A peripheral section of a layer which contributes to shell 64 isreferred to herein as an “envelope region” of the layer. In thenon-limiting example of FIGS. 9A and 9B, each of layers 62 has anenvelope region. Namely, each layer in FIGS. 9A and 9B contributes bothto the core and to the shell. However, this need not necessarily be thecase, since, for some applications, it may be desired to have the coreexposed to the environment in some regions. In these applications, atleast some of the layers do not include an envelope region. Arepresentative example of such configuration is illustrated in thecross-sectional view of FIG. 9C, showing some layers 68 which contributeto the core but not to the shell, and some layers 70 which contribute toboth the core and the shell. In some embodiments, one or more layers donot include a region with core thermo-mechanical properties and compriseonly a region with shell thermo-mechanical properties. These embodimentsare particularly useful when the structure has one or more thin parts,wherein the layers forming those parts of the structure are preferablydevoid of a core region. A representative example of such a structure isillustrated in FIG. 11, described below. Also contemplated areembodiments in which one or more layers do not include a region withshell thermo-mechanical properties and comprise only a region with corethermo-mechanical properties.

The shell can, optionally and preferably, also cover structure 60 fromabove and/or below, relative to the z direction. In these embodiments,some layers at the top most and/or bottom most parts of structure 60have at least one material property which is different from core 66. Invarious exemplary embodiments of the invention the top most and/orbottom most parts of structure 60 have the same material property asshell 64. A representative example of this embodiment is illustrated inFIG. 9D. The top/bottom shell of structure 60 may be thinner (e.g., 2times thinner) than the side shell, e.g. when the top or bottom shellcomprises a layer above or below the structure, and therefore has thesame thickness as required for layers forming the object.

A representative example of a layer 62 suitable for some embodiments ofthe present invention is illustrated in FIG. 9E. In the schematicillustration of FIG. 9E, which is not to be considered as limiting,layer 62 has a core region 902, an inner envelope region 904, at leastpartially, more preferably completely, surrounding core region 902, andan outer envelope region 906, at least partially, more preferablycompletely, surrounding inner envelope region 904. Preferably, but notnecessarily, outer envelope region 906 is the outermost region of layer62.

Core region 902 preferably comprises a combination of at least twomodeling formulations. The combination is optionally and preferablyembodied in a voxelated manner wherein some voxels that form region 902are made of one of the modeling martial formulations, other voxels aremade of another one of the modeling martial formulations, and so on. Invarious exemplary embodiments of the invention core region 902 is madeof a voxelated combination between the first modeling formulation andthe second modeling formulation described below. The voxelatedcombination can be according to any distribution by which voxelsoccupied by the first formulation are interlaced within voxels occupiedby the second formulation, such as, but not limited to, a randomdistribution.

The ratio between the number of voxels within region 902 that areoccupied by the first modeling formulation and the number of voxelswithin region 902 that are occupied by the second modeling formulationis preferably from about 0.25 to about 0.45, or from about 0.25 to about0.4, or from about 0.3 to about 0.4, e.g., about 0.33. In any embodimentof the invention, including any embodiment that includes these ratios,region 902 is optionally and preferably devoid of any material otherthan the first formulation and the second formulation described herein.

Further embodiments related to the ratio between the first modelingmaterial formulation and the second modeling material formulation areprovided hereinunder.

Inner envelope region 904 is preferably made of a single modelingformulation, for example, the first modeling formulation describedbelow. Outer envelope region 906 is preferably made of a single modelingformulation, for example, the second modeling formulation describedbelow.

The thickness of region 904, as measured within the plane of layer 62and perpendicularly to the surface of structure 60, is preferably fromabout 0.1 mm to about 4 mm, or from about 0.1 mm to about 3.5 mm, orfrom about 0.1 mm to about 3 mm, or from about 0.1 mm to about 2.5 mm,or from about 0.1 mm to about 2 mm, or from about 0.2 mm to about 1.5mm, or from about 0.3 mm to about 1.5 mm, or from about 0.4 mm to about1.5 mm, or from about 0.4 mm to about 1.4 mm or from about 0.4 mm toabout 1.3 mm or from about 0.4 mm to about 1.2 mm or from about 0.4 mmto about 1.1 mm. The thickness of region 906, as measured within theplane of layer 62 and perpendicularly to the surface of structure 60, ispreferably from about from about 150 microns to about 600 microns, orfrom about from about 150 microns to about 550 microns, or from aboutfrom about 150 microns to about 500 microns, or from about from about150 microns to about 450 microns, or from about from about 150 micronsto about 400 microns, or from about from about 150 microns to about 350microns or from about 180 microns to about 320 microns or from about 200microns to about 300 microns or from about 220 microns to about 280microns or from about 240 microns to about 260 microns.

In some embodiments of the present invention, layer 62 comprises anadditional envelope region 908 between inner envelope region 904 andouter envelope region 906. Region 904 is preferably made of acombination, e.g., voxelated combination, of two or more modelingformulations. Typically, but not exclusively, region 904 is made of avoxelated combination including the modeling formulation making region904 (the first modeling formulation in the above example) and themodeling formulation making region 906 (the second modeling formulationin the above example). It was found by the Inventors of the presentinvention that such configuration allows region 908 to serve as astitching region that bonds region 906 to region 904.

The ratio between the number of voxels within region 908 that areoccupied by the first modeling formulation and the number of voxelswithin region 902 that are occupied by the second modeling formulationis preferably from about 0.9 to about 1.1, e.g., about 1. In anyembodiment of the invention, including any embodiment that includesthese ratios, region 908 is optionally and preferably devoid of anymaterial other than the first formulation and the second formulationdescribed herein. The thickness of region 908, as measured within theplane of layer 62 and perpendicularly to the surface of structure 60, ispreferably less than the thickness of region 904 and also less than thethickness of region 906. For example, the thickness of region 908 can befrom about 70 microns to about 100 microns or from about 75 microns toabout 95 microns or from about 80 microns to about 90 microns.

In some embodiments, one or more layers do not include a core region andcomprise only envelope regions. These embodiments are particularlyuseful when the structure has one or more thin parts, wherein the layersforming those parts of the structure are preferably devoid of a coreregion. A representative example of such a structure is illustrated inFIG. 11, in which regions marked by dashed circles are devoid of core902.

FIG. 9F is a schematic illustration of a side view of structure 60 inembodiments of the invention in which at least some of the layers 62 ofstructure 60 comprise core region 902, envelope regions 904 and 906 andoptionally also an additional envelope region 908 between regions 904and 906. In these embodiments structure 60 optionally and preferablycomprises a base section 910 and/or a top section 920, each optionallyand preferably comprises a plurality of layers.

The layers of sections 910 and 920 can be arranged such that one or moreof the topmost layers 922 of top section 920 and one or more of thebottommost layers 912 of base section 910 are made of the sameformulation at envelope region 906 described above. Alternatively, ormore preferably additionally, the layers of sections 910 and 920 can bearranged such that one or more of the bottommost layers 924 of topsection 920 and one or more of the topmost layers 914 of base section910 are made of the same formulation at envelope region 904 describedabove. In some embodiments of the present invention at least one of basesection 910 and top section 920 comprises one or more intermediatelayers (respectively shown at 918, 928) that is made of the same orsimilar combination of formulations as region 908 described above.

For clarity of presentation, FIG. 9F shows a single layer for each oflayers 912, 914, 918, 922, 924 and 928, however, this need notnecessarily be the case, since, for some applications, at least one ofthese layers is embodied as a stack of layers. The number of layers ineach stack is preferably selected such that the thickness, along thebuild direction (the z direction, in the present illustration) of thestack is a proximately the same as the thickness of the respectiveenvelope region. Specifically, the number of layers in stacks 912 and922 is preferably selected such that the overall thickness of thesestacks along the build direction is approximately the same (e.g., within10%) as the thickness of outer envelope region 906 as measured in theplane of layer 62 and perpendicularly to the surface of structure 60,the number of layers in stacks 914 and 924 is preferably selected suchthat the overall thickness of these stacks along the build direction isapproximately the same (e.g., within 10%) as the thickness of innerenvelope region 904 as measured in the plane of layer 62 andperpendicularly to the surface of structure 60, and the number of layersin stacks 918 and 928 is preferably selected such that the overallthickness of these stacks along the build direction is approximately thesame (e.g., within 10%) as the thickness of additional envelope region908 as measured in the plane of layer 62 and perpendicularly to thesurface of structure 60.

The present embodiments thus provide a method of layerwise fabricationof a three-dimensional object, in which for each of at least a few(e.g., at least two or at least three or at least 10 or at least 20 orat least 40 or at least 80) of the layers or all the layers, two or moremodeling formulations are dispensed, optionally and preferably usingsystem 10 or system 110, to form a core region and at least one enveloperegion at least partially surrounding core region. Each modelingformulation is preferably dispensed by jetting it out of a plurality ofnozzles of a print head (e.g., print head 16). The dispensing isoptionally and preferably in a voxelated manner.

The core region is optionally and preferably formed from a firstmodeling formulation as well as a second modeling formulation, asdescribed herein in any of the respective embodiments. This isoptionally and preferably, but not necessarily, achieved by interlacingvoxels of the first modeling formulation and voxels of the secondmodeling formulation within the core according to a predetermined voxelratio. In some embodiments of the present invention the amount of thefirst modeling formulation is the core region is higher than 25% orhigher than 26% or higher than 27% or higher than 28% or higher than 29%or higher than 30% of a total weight of core region. In some embodimentsof the present invention the ratio between the weight of the firstmodeling formulation in the core region and the weight of the secondmodeling formulation in the core region is from about 0.1 to about 10,or from about 0.2 to about 5, or from about 0.2 to about 2, or fromabout 0.2 to about 1, or from about 0.2 to about 0.5, or from about 1 toabout 10, or from about 2 to about 10, or from about 5 to about 10.

Preferably, the two modeling formulations forming the core region areselected such that the core region, when hardened, is characterized byHDT of at least 60° C.

As used herein, HDT refers to a temperature at which the respectiveformulation or combination of formulations deforms under a predeterminedload at some certain temperature. Suitable test procedures fordetermining the HDT of a formulation or combination of formulations arethe ASTM D-648 series, particularly the ASTM D-648-06 and ASTM D-648-07methods. In various exemplary embodiments of the invention the core andshell of the structure differ in their HDT as measured by the ASTMD-648-06 method as well as their HDT as measured by the ASTM D-648-07method. In some embodiments of the present invention the core and shellof the structure differ in their HDT as measured by any method of theASTM D-648 series. In the majority of the examples herein, HDT at apressure of 0.45 MPa was used.

One or more of the envelope regions are optionally and preferably formedfrom one of the formulations, and preferably not from the otherformulation. For example, an envelope region can be formed from thefirst modeling formulation but not the second modeling formulation, orbe formed from the second modeling formulation but not the firstmodeling formulation.

Once formed, the layer including the two modeling formulations ispreferably exposed to a curing condition (e.g., curing energy) so as toharden the formulations. This is optionally and preferably executedusing hardening device 324 or radiation source 18. Alternatively, acuring condition can be exposure to the environment and/or to a chemicalreagent.

In some of any of the embodiments described herein, the buildingmaterial further comprises a support material.

In some of any of the embodiments described herein, dispensing abuilding material formulation (uncured building material) furthercomprises dispensing support material formulation(s) which form thesupport material upon application of curing energy.

Dispensing the support material formulation, in some embodiments, iseffected by inkjet printing head(s) other than the inkjet printing headsused for dispensing the first and second (and other) compositionsforming the modeling material.

In some embodiments, exposing the dispensed building material to acuring condition (e.g., curing energy) includes applying a curingcondition (e.g., curing energy) that affects curing of a supportmaterial formulation, to thereby obtain a cured support material.

In some of any of the embodiments described herein, once a buildingmaterial is cured, the method further comprises removing the curedsupport material. Any of the methods usable for removing a supportmaterial can be used, depending on the materials forming the modelingmaterial and the support material. Such methods include, for example,mechanical removal of the cured support material and/or chemical removalof the cured support material by contacting the cured support materialwith a solution in which it is dissolvable (e.g., an alkaline aqueoussolution).

As used herein, the term “curing” describes a process in which aformulation is hardened or solidifies, and is also referred to herein as“hardening”. This term encompasses polymerization of monomer(s) and/oroligomer(s) and/or cross-linking of polymeric chains (either of apolymer present before curing or of a polymeric material formed in apolymerization of the monomers or oligomers). This term alternativelyencompasses solidification of the formulation that does not involvepolymerization and/or cross-linking.

The product of a curing reaction is typically a polymeric material andin some cases a cross-linked polymeric material. This term, as usedherein, encompasses also partial curing, for example, curing of at least20% or at least 30% or at least 40% or at least 50% or at least 60% orat least 70% of the formulation, as well as 100% of the formulation.

A “curing energy” typically includes application of radiation orapplication of heat, as described herein.

A curable material or system that undergoes curing upon exposure toelectromagnetic radiation is referred to herein interchangeably as“photopolymerizable” or “photoactivatable” or “photocurable”.

When the curing energy comprises heat, the curing is also referred toherein and in the art as “thermal curing” and comprises application ofthermal energy. Applying thermal energy can be effected, for example, byheating a receiving medium onto which the layers are dispensed or achamber hosting the receiving medium, as described herein. In someembodiments, the heating is effected using a resistive heater.

In some embodiments, the heating is effected by irradiating thedispensed layers by heat-inducing radiation. Such irradiation can beeffected, for example, by means of an IR lamp or Xenon lamp, operated toemit radiation onto the deposited layer.

In some of any of the embodiments described herein, the method furthercomprises exposing the cured or solidified modeling material, eitherbefore or after removal of a support material formulation, if such hasbeen included in the building material, to a post-treatment condition.The post-treatment condition is typically aimed at further hardening thecured modeling material. In some embodiments, the post-treatment hardensa partially-cured material to thereby obtain a completely curedmaterial.

In some embodiments, the post-treatment is effected by exposure to heator radiation, as described in any of the respective embodiments herein.In some embodiments, when the condition is heat, the post-treatment canbe effected for a time period that ranges from a few minutes (e.g., 10minutes) to a few hours (e.g., 1-24 hours).

The term “post-treatment” is also referred to herein interchangeably as“post-curing treatment” or simply as “post-curing”, or as“post-hardening treatment”.

In some embodiments of the present invention there are two or moreenvelope regions surrounding or partially surrounding the core. Forexample, the core can be surrounded by an inner envelope region, and theinner envelope region can be surrounded by an outer envelope region.Preferably, but not necessarily, the lateral thickness of the innerenvelope region is from about 1 to about 5 microns, and the lateralthickness of the outer envelope region a few (e.g., from about 2 toabout 10) voxels. The lateral thickness of an envelope region is athickness that is measured within the layer, namely along a directionthat is perpendicular to the built direction.

In some embodiments of the present invention the layers are exposed toheat, during the dispensing of the formulation and/or during theexposure to curing energy. This can be executed using heating system706. The heating is preferably to a temperature which is below the HDTof the first modeling formulation, for example, at least 10° C. belowthe HDT of the first formulation. The heating can be to a temperaturewhich above the HDT of the second modeling formulation. More preferably,the heating is to a temperature which is below (e.g., at least 10° C.below) the HDT of the first modeling formulation and above an HDT ofsecond modeling formulation.

Typical temperatures to which the layer is heated, including, withoutlimitation, at least 40° C., or from about 40° C. to about 60° C.

In some embodiments of the present invention, the layer, once formed andhardened, is subjected to a post hardening treatment. Preferably, thepost hardening treatment is a thermal treatment, more preferablyheating. In a preferred embodiment, the post curing treatment includesmaintaining a temperature of at least 120° C., for a time period of atleast 1 hour.

The present embodiments contemplate several types of formulations foreach of the first and second modeling formulations that are dispensed toform the layers of the object.

In some embodiments of the present invention the first modelingformulation is characterized, when hardened, by HDT of at least 90° C.,in some embodiments of the present invention the second modelingformulation is characterized, when hardened, by Izod impact resistance(IR) value of at least 45 J/m, in some embodiments of the presentinvention the second modeling formulation is characterized, whenhardened, by HDT lower than 50° C., or lower than 45° C., and in someembodiments of the present invention a ratio between elastic moduli offirst and second modeling formulations, when hardened, is from about 2.7to about 2.9.

As used herein, the term “Izod impact resistance” refers to the loss ofenergy per unit of thickness following an impact force applied to therespective formulation or combination of formulations. Suitable testprocedures for determining the Izod impact resistance of a formulationor combination of formulations are the ASTM D-256 series, particularlythe ASTM D-256-06 series. In some embodiments of the present inventionthe core and shell of the structure differ in their Izod impactresistance value as measured by any method of the ASTM D-256-06 series.It is noted that in the standard ASTM methods there is need to machinatea notch. However, in many cases, this process cuts the shell and exposesthe core to the notch tip. Therefore, this standard method is lesspreferred for evaluating the impact resistance of a structure builtaccording to some embodiments of the present invention. Preferredprocedures for determining the impact resistance will now be described.These procedures are particularly useful when the AM includes comprisesthree-dimensional printing.

According to a first procedure, a test specimen is printed with arectangular patch made of the shelling formulation or combination offormulations. The dimensions of the patch are calculated in such waythat after the notch preparation (as required by the standard ASTMprocedure) a 0.25 mm layer of the shelling formulation or combination offormulations remains complete.

According to a second procedure, a test specimen is printed with notchinstead of cutting the notch after the specimen is printed. Theorientation of the specimen on the tray is vertical, for example, in theZ-Y plane (referred to herein as “orientation F”).

Before providing a further detailed description of the modelingformulations according to some embodiments of the present invention,attention will be given to the advantages and potential applicationsoffered thereby.

The present inventors have devised a layered manufacturing AM technologythat allows building objects with improved thermo-mechanical properties,even when those properties are not possessed by any one of the modelingformulations used for fabricating the object. For example, embodimentsof the present invention provide AM structures, more preferablystructures manufactured by jetting two modeling formulations via 3Dinkjet printing technology, with high temperature resistance as well ashigh toughness. Embodiments of the present invention also allowfabricating structures with, for example, high temperature resistance aswell as low curling.

The present embodiments thus provide objects that are preferablyfabricated by AM, more preferably by 3D inkjet printing technology, andthat are characterized by HDT of at least 100° C., or at least 130° C.,or at least 140° C. The present embodiments can provide objects that arepreferably fabricated by AM, more preferably by 3D inkjet printingtechnology, and that are characterized by Izod notch impact resistanceof at least100 J/m or at least110 J/m or at least120 J/m or at least130J/m. The present embodiments can provide objects that are preferablyfabricated by AM, more preferably by 3D inkjet printing technology, andthat feature curling of less than 4 mm, or less than 3 mm.

In any of the methods and systems described herein, at least a firstmodeling material formulation and a second modeling material formulationare utilized.

The present inventors found that the combination of a first modelingmaterial formulation and a second modeling material formulation can beused to provide a desired stiffness and strength. For example,increasing the content of the first formulation in the core can increasethe strength and stiffness of the fabricated object without increasingthe HDT. The HDT, in turn, can be controlled by varying the thickness ofa shell. For example, when the shell is made of the first formulationbut not from the second formulation, the HDT can be increased byincreasing the lateral thickness of the shell.

According to some of any of the embodiments described herein the firstmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, heat deflection temperature (HDT) of atleast 90° C.

According to some of any of the embodiments described herein, the secondmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, Izod impact resistance (IR) value of atleast 45 J/m.

According to some of any of the embodiments described herein the firstmodeling formulation is characterized by, features, or selected suchthat it features, when hardened, heat deflection temperature (HDT) of atleast 90° C., and the second modeling formulation is characterized by,features, or selected such that it features, when hardened, Izod impactresistance (IR) value of at least 45 J/m.

In some embodiments, the HDT of the first formulation, when hardened, isat least 100° C., or at least 110° C., or at least 120° C., or at least130° C., or at least 135° C., or at least 140° C., or higher.

In some embodiments, the Impact resistance (Izod impact resistance) ofthe second formulation is at least 47, or at least 48, or a least 49, orat least 50, or at least 51, or at least 52, or at least 53, or at least54, or at least 55, J/m, or higher.

In some of any of the embodiments described herein, the second modelingformulation is characterized by, or features, or selected so as tofeature, when hardened, by HDT lower than 50° C., or lower than 45° C.

In some embodiments the HDT of the second formulation, when hardened,ranges from 30 to 50° C., or from 35 to 50° C., or from 38 to 50° C., orfrom 40 to 50° C., or from 40 to 48° C., or from 40 to 45° C., or from30 to 45° C., or from 35 to 45° C., including any intermediate value andsubranges therebetween.

In some of any of the embodiments described herein, the first and secondformulations are characterized by, feature, or selected so as tofeature, when hardened, a ratio between elastic moduli which is lessthan 3.

In some embodiments, the ratio ranges from 2.7 to 2.9.

According to some of any of the embodiments described herein, each ofthe modeling material formulations comprises one or more curablematerials.

Herein throughout, a “curable material” or a “solidifiable” is acompound (e.g., monomeric or oligomeric or polymeric compound) which,when exposed to a curing condition (e.g., curing energy), as describedherein, solidifies or hardens to form a cured modeling material asdefined herein. Curable materials are typically polymerizable materials,which undergo polymerization and/or cross-linking when exposed tosuitable energy source.

In some of any of the embodiments described herein, a curable materialis a photopolymerizable material, which polymerizes or undergoescross-linking upon exposure to radiation, as described herein, and insome embodiments the curable material is a UV-curable material, whichpolymerizes or undergoes cross-linking upon exposure to UV-visradiation, as described herein.

In some embodiments, a curable material as described herein is apolymerizable material that polymerizes via photo-induced radicalpolymerization.

In some of any of the embodiments described herein, a curable materialcan be a monomer, an oligomer or a short-chain polymer, each beingpolymerizable as described herein.

In some of any of the embodiments described herein, when a curablematerial is exposed to curing energy (e.g., radiation), it polymerizesby any one, or combination, of chain elongation and cross-linking.

In some of any of the embodiments described herein, a curable materialis a monomer or a mixture of monomers which can form a polymericmodeling material upon a polymerization reaction, when exposed to curingenergy at which the polymerization reaction occurs. Such curablematerials are also referred to herein as monomeric curable materials.

In some of any of the embodiments described herein, a curable materialis an oligomer or a mixture of oligomers which can form a polymericmodeling material upon a polymerization reaction, when exposed to curingenergy at which the polymerization reaction occurs. Such curablematerials are also referred to herein as oligomeric curable materials.

In some of any of the embodiments described herein, a curable material,whether monomeric or oligomeric, can be a mono-functional curablematerial or a multi-functional curable material.

Herein, a mono-functional curable material comprises one functionalgroup that can undergo polymerization when exposed to curing energy(e.g., radiation).

A multi-functional curable material comprises two or more, e.g., 2, 3, 4or more, functional groups that can undergo polymerization when exposedto curing energy. Multi-functional curable materials can be, forexample, di-functional, tri-functional or tetra-functional curablematerials, which comprise 2, 3 or 4 groups that can undergopolymerization, respectively. The two or more functional groups in amulti-functional curable material are typically linked to one another bya linking moiety, as defined herein. When the linking moiety is anoligomeric moiety, the multi-functional group is an oligomericmulti-functional curable material.

In some embodiments, at least some, or each of, the curable materials ineach of the first and second formulations, are (meth)acrylic materials.

Herein throughout, the term “(meth)acrylic” or “(meth)acrylate” anddiversions thereof encompasses both acrylic and methacrylic materials.

Acrylic and methacrylic materials encompass materials bearing one ormore acrylate, methacrylate, acrylamide and/or methacrylamide group.

Each of the curable materials can independently be a monomer, anoligomer or a polymer (which may undergo, for example, cross-linking,when cured).

Each of the curable materials can independently be a mono-functional, ormulti-functional material.

The curable materials included in the first and second formulationsdescribed herein may be defined by the properties provided by eachmaterial, when hardened. That is, the materials are defined byproperties of a material formed upon exposure to curing energy, namely,upon polymerization. These properties (e.g., Tg), are of a polymericmaterial formed upon curing any of the described curable materialsalone.

In some of any of the embodiments described herein, the first modelingformulation comprises:

at least one curable acrylic monomer characterized, when hardened, by Tgof at least 85° C.;

at least one curable methacrylic monomer characterized, when hardened,by Tg of at least 150° C.; and

at least one curable (meth)acrylic oligomer, characterized, whenhardened, by Tg of at least 50° C.

In some of any of the embodiments described herein, a concentration ofthe curable methacrylic monomer is at least 35% by weight of the totalweight of the first modeling formulation. In some embodiments, aconcentration of the curable methacrylic monomer in the firstformulation ranges from 35 to 60%, or from 35 to 50%, or from 35 to 40%,by weight, of the total weight of the first modeling formulation,including any intermediate values and subranges therebetween.

In some of any of the embodiments described herein, the curablemethacrylic monomer in the first modeling formulation is characterizedby a curing rate lower than 4400 mW/minute, or lower than 4000mW/minute, or lower than 3800, or lower than 3500, or lower than 3000,or lower than 2800 mW/minute, or lower. In some embodiments, the curingrate is determined by Photo DSC measurements, and is defined as theslope of the curve recorded for mW as a function of time, for example,as described in the Examples section that follows.

In some embodiments, the high Tg methacrylate monomer is a difunctionalmethacrylate, and in some embodiments, it comprises one or morehydrocarbon chain(s) and/or ring(s) of at least 6, or at least 8, carbonatoms. Monofunctional, or other multifunctional methacrylate monomers,featuring such high-carbon chains and/or rings are also contemplated.

A non-limiting, exemplary methacrylic monomer which is characterized,when hardened, by Tg higher than 150° C., and/or by a curing rate asdescribed herein, is SR 843 (Tricyclodecanedimethanol dimethacrylate(TCDDMDMA)). An additional exemplary such material is sold under thebrand name SR-423D. See, for example Table 1.

In some embodiments, acrylic monomers characterized, when hardened, byTg higher than 85° C. include monofunctional, difunctional, othermultifunctional acrylate monomers, and any mixture thereof. In someembodiments, the Tg of the acrylate monomers ranges from 86 to about300° C.

The acrylate monomers featuring such Tg can be, for example, commonlyused monofunctional acrylate monomers such as ACMO and IBOA;multifunctional acrylate monomers such as, for example, Tris(2-hydroxyethyl) isocyanurate triacrylate (THEICTA), commerciallyavailable under the name SE368; short-chain alkylene glycol-containing(ethoxylated) difunctional and trifunctional acrylate monomers such as,for example, DPGDA (commercially available under the name SR508),ethoxylated 3 trimethylolpropane triacrylate (TMP3EOTA), commerciallyavailable under the name SR454, and long-chain or high-carbon ringmultifunctional acrylate monomers such as, for example,Tricyclodecanedimethanol diacrylate (TCDDMDA), commercially availableunder the name SR833S.

Exemplary acrylic monomers characterized, when hardened, by Tg higherthan 85° C. include, but are not limited to, those presented in Table 1hereinbelow. Any other acrylic monomer featuring the indicated Tg iscontemplated. Those skilled in the art would readily recognizeadditional acrylate monomers featuring Tg higher than 85° C.

The acrylic monomer featuring the indicated Tg, when hardened, can be amixture of two or more such monomers.

In some embodiments, a concentration of each of the curable acrylicmonomers in the first formulation ranges from 5 to 40% by weight of thetotal weight of the first modeling formulation, including anyintermediate values and subranges therebetween. In some embodiments,when two or more such acrylic monomers are present in the formulation,the total concentration of such monomers ranges from 10 to 60, or from10 to 50, or from 1o to 40% by weight of the total weight of the firstmodeling formulation, including any intermediate values and subrangestherebetween.

In some embodiments, the (meth)acrylic oligomer is characterized, whenhardened, by Tg of at least 50° C., is or comprises an acrylic oligomer,or, alternatively a mixture of two or more acrylic monomers or of one ormore acrylic monomers and one or more methacrylic monomers.

Exemplary such oligomers include, but are not limited to, polyesterurethane acrylates, epoxy acrylates, modified (e.g., amine modified)epoxy acrylate and the like. Non-limiting examples are presented inTable 1 below. Any other acrylic oligomers featuring the indicated Tg iscontemplated. Those skilled in the art would readily recognizeadditional acrylate oligomers featuring Tg higher than 50° C.

In some embodiments, a total concentration of the (meth)acrylicoligomer(s) is the first modeling formulation ranges from 10 to 60%, orfrom 10 to 50%, or from 10 to 40%, or from 10 to 30%, or from 10 to 20%,by weight of the total weight of the first modeling formulation,including any intermediate values and subranges therebetween.

In some embodiments, the first modeling formulation may further compriseat least one curable (meth)acrylic monomer which provides, whenhardened, a flexible material, characterized by Tg lower than 0° C., orlower than −10° C., or lower than −20° C.

In some embodiments, the (meth)acrylic monomer characterized, whenhardened, by Tg lower than −10 or −20° C., is or comprises an acrylicmonomer, or, alternatively a mixture of two or more acrylic monomers orof one or more acrylic monomers and one or more methacrylic monomers.

Acrylic and methacrylic monomers featuring such low Tg include, forexample, ethoxylated monofunctional, or preferably multifunctional(e.g., difunctional or trifunctional), as described herein in any of therespective embodiments.

Exemplary such flexible acrylic monomers are presented in Table 1 below.Any other flexible acrylic (or methacrylic) monomers are contemplated.Those skilled in the art would readily recognize additional acrylatemonomers featuring low Tg as indicated.

In some embodiments, a concentration of the flexible (meth)acrylicmonomer, if present, ranges from 4 to 30, or from 4 to 25, or from 4 to20, or from 4 to 15, or from 4.5 to 13.5 weight percents, of the totalweight of the formulation, including any intermediate values andsubranges therebetween.

In some embodiments, the flexible monomer is a multi-functionalethoxylated monomer as described herein, in which each of the(meth)acrylate groups are linked to an alkylene glycol group or chain,and the alkylene glycol groups or chains are linked to one anotherthrough a branching unit, such as, for example, a branched alkyl,cycloalkyl, aryl (e.g., bisphenol A), etc., as described in furtherdetail hereinunder.

In some of any of the embodiments described herein, the first modelingformulation further comprises an additional curable (meth)acrylicmonomer which provides, when hardened, a flexible material,characterized by Tg lower than 0° C., or lower than −10° C., or lowerthan −20° C.

In some embodiments, the additional flexible monomer is a di-functionalmonomer which comprises an alkylene glycol chain (a poly(alkyleneglycol, as defined herein) that terminates at both ends by an acrylateor methacrylate group.

In some embodiments, the poly(alkylene glycol) chain features at least5, preferably at least 10, e.g., from 10 to 15, alkylene glycol groups.

In some embodiments, the concentration of the additional flexiblemonomer as described herein ranges from 5 to 20, or from 5 to 15, weightpercents, of the total weight of the formulation, including anyintermediate values and subranges therebetween.

In some of any of the embodiments described herein, the firstformulation comprises, as a flexible monomer having a low Tg asindicated herein only the material described herein as “an additionalflexible monomer”.

In some embodiments, the first formulation is used in combination withany second formulation that features, when hardened, Impact resistancevalue and/or HDT, as defined herein.

In some embodiments, the second modeling formulation comprises:

at least one curable (meth)acrylic, preferably acrylic, monomercharacterized, when hardened, by Tg of at least70° C.;

at least one curable (meth)acrylic, preferably acrylic, oligomercharacterized, when hardened, by Tg of at least 10° C.; and

at least one curable (meth)acrylic, preferably acrylic, ethoxylatedmonomer.

Herein, an “ethoxylated” material describes an acrylic or methacryliccompound which comprises one or more alkylene glycol groups, or,preferably, one or more alkylene glycol chains, as defined herein.Ethoxylated (meth)acrylate materials can be monofunctional, or,preferably, multifunctional, namely, difunctional, trifunctional,tetrafunctional, etc.

In multifunctional materials, typically, each of the (meth)acrylategroups are linked to an alkylene glycol group or chain, and the alkyleneglycol groups or chains are linked to one another through a branchingunit, such as, for example, a branched alkyl, cycloalkyl, aryl (e.g.,bisphenol A), etc.

In some embodiments, the ethoxylated material comprises at least 5ethoxylated groups, that is, at least 5 alkylene glycol moieties orgroups. Some or all of the alkylene glycol groups can be linked to oneanother to form an alkylene glycol chain. For example, an ethoxylatedmaterial that comprises 30 ethoxylated groups can comprise a chain of 30alkylene glycol groups linked to one another, two chains, each, forexample, of 15 alkylene glycol moieties linked to one another, the twochains linked to one another via a branching moiety, or three chains,each, for example, of 10 alkylene glycol groups linked to one another,the three chains linked to one another via a branching moiety. Shorterand longer chains are also contemplated.

In some embodiments, the ethoxylated material comprises at least 8, orat least 10, or at least 12, or at least 15, or at least 18, or at least20, or at least 25, or at least 30 ethoxylated (alkylene glycol) groups.The ethoxylated material can comprise one, two or more poly(alkyleneglycol) chains, of any length, as long as the total number of thealkylene glycol groups is as indicated.

In some embodiments, the ethoxylated material is a flexible material,characterized, when hardened, by Tg lower than 0° C., preferably lowerthan −10° C. or lower than −20° C.

In some of any of the embodiments described herein, the ethoxylatedcurable monomer is characterized by a viscosity at room temperaturelower than 1000 centipoises; and/or by a molecular weight of at least500 grams/mol.

Non-limiting examples of ethoxylated materials suitable for inclusion inthe second modeling formulation are presented in Table 6A hereinbelow.

In some of any of the embodiments described herein, the ethoxylatedmaterial is a trifunctional (meth)acrylate monomer. Exemplary suchtrifunctional monomers are also presented in Table 6A below. Otherflexible, ethoxylated trifunctional monomers are contemplated. Thoseskilled in the art would readily recognize other trifunctional monomersfeaturing the indicated properties.

In some of any of the embodiments described herein, a concentration ofthe ethoxylated curable monomer in the second modeling formulation is atleast 5, or at least 10, weight percents of the total weight of thesecond modeling formulation.

In some embodiments, the concentration of the ethoxylated curablematerial ranges from 5 to 50, or from 5 to 40, or from 10 to 50, or from10 to 40, or from 10 to 30, % by weight of the total weight of thesecond modeling formulation, including any intermediate values andsubranges therebetween.

In addition to the ethoxylated curable material, the second formulationcomprises at least one curable (meth)acrylic, preferably acrylic,monomer characterized, when hardened, by Tg of at least 70° C.; and atleast one curable (meth)acrylic, preferably acrylic, oligomercharacterized, when hardened, by Tg of at least 10° C.

In some embodiments, the curable (meth)acrylic, preferably acrylic,monomer characterized, when hardened, by Tg of at least 70° C., ischaracterized by Tg of at least 85° C., when hardened, and includemonofunctional and multifunctional monomers, and any mixture of suchmonomers, as described herein. Exemplary such monomers are presented inTable 5 below. Any other monomers featuring the indicated Tg arecontemplated.

In some embodiments, the total concentration of such curable(meth)acrylic monomer(s) in the second modeling formulation ranges from10 to 50% by weight of the total weight of the second modelingformulation.

Curable (meth)acrylic oligomers characterized, when hardened, by Tg ofat least 10° C., include monofunctional, and preferably multifunctionaloligomers such as, but not limited to, polyester urethane acrylates,epoxy acrylates, modified epoxy acrylates, etc. Those skilled in the artwould readily recognize oligomers featuring the indicated Tg. Exemplarysuch oligomers are described hereinabove, and some are presented inTable 5 hereinbelow. Any other oligomers are contemplated.

In some embodiments, the total concentration of the curable(meth)acrylic oligomer in the second modeling formulation ranges from 10to 50% by weight of the total weight of the second modeling formulation,including any intermediate values and subranges therebetween.

In some of any of the embodiments described herein, each of the first,second, and optionally other modeling material formulationsindependently comprises a photoinitiator, for initiating thepolymerization or cross-linking (curing) upon exposure to curing energy(e.g., radiation).

In some embodiments, the photoinitiator is a free-radical initiator.

A free radical photoinitiator may be any compound that produces a freeradical on exposure to radiation such as ultraviolet or visibleradiation and thereby initiates a polymerization reaction. Non-limitingexamples of suitable photoinitiators include benzophenones (aromaticketones) such as benzophenone, methyl benzophenone, Michler's ketone andxanthones; acylphosphine oxide type photo-initiators such as2,4,6-trimethylbenzolydiphenyl phosphine oxide (TMPO),2,4,6-trimethylbenzoylethoxyphenyl phosphine oxide (TEPO), monoacylphosphine oxides (MAPO's) and bisacylphosphine oxides (BAPO's); benzoinsand benzoin alkyl ethers such as benzoin, benzoin methyl ether andbenzoin isopropyl ether and the like. Examples of photoinitiators arealpha-amino ketone, alpha-hydroxy ketone, monoacyl phosphine oxides(MAPO's) and bisacylphosphine oxide (BAPO's).

A free-radical photo-initiator may be used alone or in combination witha co-initiator. Co-initiators are used with initiators that need asecond molecule to produce a radical that is active in the UV-systems.Benzophenone is an example of a photoinitiator that requires a secondmolecule, such as an amine, to produce a curable radical. Afterabsorbing radiation, benzophenone reacts with a ternary amine byhydrogen abstraction, to generate an alpha-amino radical which initiatespolymerization of acrylates. Non-limiting example of a class ofco-initiators are alkanolamines such as triethylamine,methyldiethanolamine and triethanolamine.

In some embodiments, a concentration of the initiator in the firstand/or the second modeling material formulation independently rangesfrom 0.5 to 5%, or from 1 to 5%, or from 2 to 5%, by weight of the totalweight of the respective formulation.

In some of any of the embodiments described herein, the first and/orsecond modeling material formulation independently further comprises oneor more additional materials, which are referred to herein also asnon-reactive materials (non-curable materials).

Such agents include, for example, surface active agents (surfactants),inhibitors, antioxidants, fillers, pigments, dyes, and/or dispersants.

Surface-active agents may be used to reduce the surface tension of theformulation to the value required for jetting or for printing process,which is typically around 30 dyne/cm. Such agents include siliconematerials, for example, organic polysiloxanes such as PDMS andderivatives therefore, such as those commercially available as BYK typesurfactants.

Suitable dispersants (dispersing agents) can also be silicone materials,for example, organic polysiloxanes such as PDMS and derivativestherefore, such as those commercially available as BYK type surfactants.

Suitable stabilizers (stabilizing agents) include, for example, thermalstabilizers, which stabilize the formulation at high temperatures.

The term “filler” describes an inert material that modifies theproperties of a polymeric material and/or adjusts a quality of the endproducts. The filler may be an inorganic particle, for example calciumcarbonate, silica, and clay.

Fillers may be added to the modeling formulation in order to reduceshrinkage during polymerization or during cooling, for example, toreduce the coefficient of thermal expansion, increase strength, increasethermal stability, reduce cost and/or adopt rheological properties.Nanoparticles fillers are typically useful in applications requiring lowviscosity such as ink-jet applications.

In some embodiments, a concentration of each of a surfactant and/or adispersant and/or a stabilizer and/or a filler, if present, ranges from0.01 to 2%, or from 0.01 to 1%, by weight, of the total weight of therespective formulation. Dispersants are typically used at aconcentration that ranges from 0.01 to 0.1%, or from 0.01 to 0.05%, byweight, of the total weight of the respective formulation.

In some embodiments, the first and/or second formulation furthercomprises an inhibitor. The inhibitor is included for preventing orreducing curing before exposure to curing energy. Suitable inhibitorsinclude, for example, those commercially available as the Genorad type,or as MEHQ. Any other suitable inhibitors are contemplated.

The pigments can be organic and/or inorganic and/or metallic pigments,and in some embodiments the pigments are nanoscale pigments, whichinclude nanoparticles.

Exemplary inorganic pigments include nanoparticles of titanium oxide,and/or of zinc oxide and/or of silica. Exemplary organic pigmentsinclude nanosized carbon black.

In some embodiments, the pigment's concentration ranges from 0.1 to 2%by weight, or from 0.1 to 1.5%, by weight, of the total weight of therespective formulation.

In some embodiments, the first formulation comprises a pigment.

In some embodiments, combinations of white pigments and dyes are used toprepare colored cured materials.

The dye may be any of a broad class of solvent soluble dyes. Somenon-limiting examples are azo dyes which are yellow, orange, brown andred; anthraquinone and triarylmethane dyes which are green and blue; andazine dye which is black.

Any of the first and/or second formulations described herein, in any ofthe respective embodiments and any combination thereof, can be providedwithin a kit, in which the first and second formulations areindividually packaged.

In some embodiments, all the components of each formulation are packagedtogether. In some of these embodiments, the formulations are packaged ina packaging material which protects the formulations from exposure tolight or any other radiation and/or comprise an inhibitor.

In some embodiments, the initiator is packaged separately from othercomponents of each formulation, and the kit comprises instructions toadd the initiator to the respective formulation.

The present inventors have devised a technique that further reduces thecurling effect. In this technique, a structure, referred to herein as “apedestal” is dispensed directly on the tray, and the layers that make upthe object are thereafter dispensed on the pedestal. This embodiment isillustrated in FIGS. 10A-10B.

FIG. 10A shows a side view of a pedestal 202 on tray 360 wherein thelayers of an object 200 are dispensed on pedestal 202. Object 200 cancomprise, or be, a shelled structure (e.g., structure 60), made of thefirst and second modeling formulations as further detailed hereinabove.Alternatively, object 200 can be a non-shelled structure, or a shelledstructure (e.g., structure 60), made of other modeling formulation, suchas a commercially available modeling formulation.

Pedestal 202 optionally and preferably serves to ease the removal ofobject from the printing tray and thus may help prevent deformation bymanual or mechanical damage. Pedestal 202 can also improve the object'saccuracy in the Z direction (height), and/or may improve an object'saccuracy in the X-Y directions.

Pedestal 202 preferably comprises a support formulation that includes asupport material. Preferably the support formulation is soluble inliquid, e.g., in water. In various exemplary embodiments of theinvention pedestal 202 comprises a combination of support formulationand modeling formulation (e.g., any of the first and second modelingformulations described herein). Preferably, the modeling formulationwithin pedestal 202 is of low Izod impact resistance, for example, lessthan 40 J/m. The advantage of this embodiment is that it reduces thetendency of the pedestal to lift from the tray.

Inaccuracies in Z may occur at the lowest layers of the printed object.This may be because the top surface of the tray at Z start level (the Zlevel of the tray when printing starts) may not be exactly at a heightwhich enables the leveling device to reach and thus level the firstlayers deposited in the printing process, when the leveling device maybe at its lowest point (e.g., because of inaccuracy in adjustmentsand/or incomplete flatness and horizon of the tray). As a result, thelower layers of object 200 may not be leveled by the leveling device andtherefore their thickness may be greater than the designed layerthickness, therefore increasing the height of object 200 as printed incontrast to the object as designed. The use of pedestal 202 under thelowest point of the object solves this problem by specifying that theheight at which the printing of the actual object starts may be theheight at which the pedestal itself may be significantly leveled by theleveling device.

In various exemplary embodiments of the invention pedestal 202 has acore-shell structure, in which the shell comprises spaced pillars ofmodeling formulation with support formulation in-between, and the corecomprises only soluble (e.g., water soluble) support formulation, and isdevoid of any non-soluble modeling formulation. These embodiments areillustrated in FIG. 10B which is a top view illustration of a typicallayer of pedestal 202, having a pedestal core (shown as a core region208 in FIG. 10B) and pedestal shell (shown as an envelope region 210 inFIG. 10B). The support formulation is shown at 204 (patterned filling)and the modeling formulation pillars are shown at 206 (white filling).

The advantage of forming a pedestal with core-shell structure as definedabove is that it solves the problems of Z inaccuracies and curling whileminimizing the use of modeling formulation, which is typically moreexpensive, and tends to leave visible remnants at the bottom of object200.

Herein throughout, the phrase “elastic modulus” refers to Young'smodulus, as determined by response of a material to application oftensile stress.

The elastic modulus is determined as the gradient of stress as afunction of strain over ranges of stress and strain wherein stress is alinear function of strain (e.g., from a stress and strain of zero, tothe elastic proportionality limit, and optionally from zero strain to astrain which is no more than 50% of the elongation at failure).

The elongation at failure, which is also referred to herein and in theart as elongation at break, ε_(R), is determined as the maximal strain(elongation) which can occur (upon application of tensile stress equalto the ultimate tensile strength) before failure of the tested materialoccurs (e.g., as rupture or necking).

Recovery is determined by releasing the tensile stress after subjectingthe tested material as the ratio of the decrease in length to a priorstrain after a material (e.g., elastic layer) is subjected to a priorstrain which is almost equal to the elongation at failure (optionallyabout 90% of the elongation at failure, optionally about 95% of theelongation at failure, optionally about 98% of the elongation atfailure, optionally about 99% of the elongation at failure, wherein theelongation at failure can be determined using an equivalent sample).Thus, for example, a material extended to an elongation at failure whichis 200%, and which upon release of tensile stress returns to a statecharacterized by a strain of 20% relative to the original length, wouldbe characterized as having a recovery of 90% (i.e., 200%-20% divided by200%).

Herein, “Tg” refers to glass transition temperature defined as thelocation of the local maximum of the E″ curve, where E″ is the lossmodulus of the material as a function of the temperature. Broadlyspeaking, as the temperature is raised within a range of temperaturescontaining the Tg temperature, the state of a material, particularly apolymeric material, gradually changes from a glassy state into a rubberystate.

Herein, “Tg range” is a temperature range at which the E″ value is atleast half its value (e.g., can be up to its value) at the Tgtemperature as defined above.

Without wishing to be bound to any particular theory, it is assumed thatthe state of a polymeric material gradually changes from the glassystate into the rubbery within the Tg range as defined above. The lowesttemperature of the Tg range is referred to herein as Tg(low) and thehighest temperature of the Tg range is referred to herein as Tg(high).

In any of the embodiments described herein, the term “temperature higherthan Tg” means a temperature that is higher than the Tg temperature, or,more preferably a temperature that is higher than Tg(high).

Herein, “Tg sum” describes the total calculated Tg of a formulation(e.g., a modeling formulation), as calculated by summing individual Tgvalues of polymeric components of the formulation. The summation isoptionally and preferably a weight sum, wherein each Tg value ismultiplied by the relative amount (e.g., weight percentage) of therespective polymeric components of first modeling formulation. Thepolymeric components can be the respective curable components thatprovide a polymeric component featuring a Tg, or non-curable polymericcomponents added to the formulation.

Herein throughout, the term “object” describes a final product of theadditive manufacturing. This term refers to the product obtained by amethod as described herein, after removal of the support material, ifsuch has been used as part of the building material. The “object”therefore essentially consists (at least 95 weight percents) of ahardened (e.g., cured) modeling material.

The term “object” as used herein throughout refers to a whole object ora part thereof.

Herein throughout, the phrases “building material formulation”, “uncuredbuilding material”, “uncured building material formulation”, “buildingmaterial” and other variations therefore, collectively describe thematerials that are dispensed to sequentially form the layers, asdescribed herein. This phrase encompasses uncured materials dispensed soas to form the object, namely, one or more uncured modeling materialformulation(s), and uncured materials dispensed so as to form thesupport, namely uncured support material formulations.

Herein throughout, the phrase “cured modeling material” or “hardenedmodeling material” describes the part of the building material thatforms the object, as defined herein, upon exposing the dispensedbuilding material to curing, and, optionally, if a support material hasbeen dispensed, also upon removal of the cured support material, asdescribed herein. The cured modeling material can be a single curedmaterial or a mixture of two or more cured materials, depending on themodeling material formulations used in the method, as described herein.

The phrase “cured modeling material” or “cured modeling materialformulation” can be regarded as a cured building material wherein thebuilding material consists only of a modeling material formulation (andnot of a support material formulation). That is, this phrase refers tothe portion of the building material, which is used to provide the finalobject.

Herein throughout, the phrase “modeling material formulation”, which isalso referred to herein interchangeably as “modeling formulation”,“model formulation” “model material formulation” or simply as“formulation”, describes a part or all of the building material which isdispensed so as to form the object, as described herein. The modelingmaterial formulation is an uncured modeling formulation (unlessspecifically indicated otherwise), which, upon exposure to curingenergy, forms the object or a part thereof.

In some embodiments of the present invention, a modeling materialformulation is formulated for use in three-dimensional inkjet printingand is able to form a three-dimensional object on its own, i.e., withouthaving to be mixed or combined with any other substance.

The phrase “digital materials”, as used herein and in the art, describesa combination of two or more materials on a microscopic scale or voxellevel such that the printed zones of a specific material are at thelevel of few voxels, or at a level of a voxel block. Such digitalmaterials may exhibit new properties that are affected by the selectionof types of materials and/or the ratio and relative spatial distributionof two or more materials.

In exemplary digital materials, the modeling material of each voxel orvoxel block, obtained upon curing, is independent of the modelingmaterial of a neighboring voxel or voxel block, obtained upon curing,such that each voxel or voxel block may result in a different modelmaterial and the new properties of the whole part are a result of aspatial combination, on the voxel level, of several different modelmaterials.

Herein throughout, whenever the expression “at the voxel level” is usedin the context of a different material and/or properties, it is meant toinclude differences between voxel blocks, as well as differences betweenvoxels or groups of few voxels. In preferred embodiments, the propertiesof the whole part are a result of a spatial combination, on the voxelblock level, of several different model materials.

The term “branching unit” as used herein describes a multi-radical,preferably aliphatic or alicyclic moiety, and optionally an aryl orheteroaryl moiety. By “multi-radical” it is meant that the branchingunit has two or more attachment points such that it links between two ormore atoms and/or groups or moieties.

That is, the branching unit is a chemical moiety that, when attached toa single position, group or atom of a substance, creates two or morefunctional groups that are linked to this single position, group oratom, and thus “branches” a single functionality into two or morefunctionalities.

In some embodiments, the branching unit is derived from a chemicalmoiety that has two, three or more functional groups. In someembodiments, the branching unit is a branched alkyl or a branchedlinking moiety as described herein.

As used herein the term “about” refers to ±10% or ±5%.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

As used herein, the term “amine” describes both a —NR′R″ group and a—NR′— group, wherein R′ and R″ are each independently hydrogen, alkyl,cycloalkyl, aryl, as these terms are defined hereinbelow.

The amine group can therefore be a primary amine, where both R′ and R″are hydrogen, a secondary amine, where R′ is hydrogen and R″ is alkyl,cycloalkyl or aryl, or a tertiary amine, where each of R′ and R″ isindependently alkyl, cycloalkyl or aryl.

Alternatively, R′ and R″ can each independently be hydroxyalkyl,trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl,heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate,hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano,nitro, azo, sulfonamide, carbonyl, C-carboxylate, O-carboxylate,N-thiocarbamate, O-thiocarbamate, urea, thiourea, N-carbamate,O-carbamate, C-amide, N-amide, guanyl, guanidine and hydrazine.

The term “amine” is used herein to describe a —NR′R″ group in caseswhere the amine is an end group, as defined hereinunder, and is usedherein to describe a —NR′— group in cases where the amine is a linkinggroup or is or part of a linking moiety.

The term “alkyl” describes a saturated aliphatic hydrocarbon includingstraight chain and branched chain groups. Preferably, the alkyl grouphas 1 to 30, or 1 to 20 carbon atoms. Whenever a numerical range; e.g.,“1-20”, is stated herein, it implies that the group, in this case thealkyl group, may contain 1 carbon atom, 2 carbon atoms, 3 carbon atoms,etc., up to and including 20 carbon atoms. The alkyl group may besubstituted or unsubstituted. Substituted alkyl may have one or moresubstituents, whereby each substituent group can independently be, forexample, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl,heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine.

The alkyl group can be an end group, as this phrase is definedhereinabove, wherein it is attached to a single adjacent atom, or alinking group, as this phrase is defined hereinabove, which connects twoor more moieties via at least two carbons in its chain. When the alkylis a linking group, it is also referred to herein as “alkylene” or“alkylene chain”.

Herein, a C(1-4) alkyl, substituted by a hydrophilic group, as definedherein, is included under the phrase “hydrophilic group” herein.

Alkene and Alkyne, as used herein, are an alkyl, as defined herein,which contains one or more double bond or triple bond, respectively.

The term “cycloalkyl” describes an all-carbon monocyclic ring or fusedrings (i.e., rings which share an adjacent pair of carbon atoms) groupwhere one or more of the rings does not have a completely conjugatedpi-electron system. Examples include, without limitation, cyclohexane,adamantine, norbornyl, isobornyl, and the like. The cycloalkyl group maybe substituted or unsubstituted. Substituted cycloalkyl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The cycloalkyl group can be an end group, as this phrase isdefined hereinabove, wherein it is attached to a single adjacent atom,or a linking group, as this phrase is defined hereinabove, connectingtwo or more moieties at two or more positions thereof.

Cycloalkyls of 1-6 carbon atoms, substituted by two or more hydrophilicgroups, as defined herein, is included under the phrase “hydrophilicgroup” herein.

The term “heteroalicyclic” describes a monocyclic or fused ring grouphaving in the ring(s) one or more atoms such as nitrogen, oxygen andsulfur. The rings may also have one or more double bonds. However, therings do not have a completely conjugated pi-electron system.Representative examples are piperidine, piperazine, tetrahydrofurane,tetrahydropyrane, morpholino, oxalidine, and the like.

The heteroalicyclic may be substituted or unsubstituted. Substitutedheteroalicyclic may have one or more substituents, whereby eachsubstituent group can independently be, for example, hydroxyalkyl,trihaloalkyl, cycloalkyl, alkenyl, alkynyl, aryl, heteroaryl,heteroalicyclic, amine, halide, sulfonate, sulfoxide, phosphonate,hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy, thioaryloxy, cyano,nitro, azo, sulfonamide, C-carboxylate, O-carboxylate, N-thiocarbamate,O-thiocarbamate, urea, thiourea, O-carbamate, N-carbamate, C-amide,N-amide, guanyl, guanidine and hydrazine. The heteroalicyclic group canbe an end group, as this phrase is defined hereinabove, where it isattached to a single adjacent atom, or a linking group, as this phraseis defined hereinabove, connecting two or more moieties at two or morepositions thereof.

A heteroalicyclic group which includes one or more of electron-donatingatoms such as nitrogen and oxygen, and in which a numeral ratio ofcarbon atoms to heteroatoms is 5:1 or lower, is included under thephrase “hydrophilic group” herein.

The term “aryl” describes an all-carbon monocyclic or fused-ringpolycyclic (i.e., rings which share adjacent pairs of carbon atoms)groups having a completely conjugated pi-electron system. The aryl groupmay be substituted or unsubstituted. Substituted aryl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,N-carbamate, O-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The aryl group can be an end group, as this term is definedhereinabove, wherein it is attached to a single adjacent atom, or alinking group, as this term is defined hereinabove, connecting two ormore moieties at two or more positions thereof.

The term “heteroaryl” describes a monocyclic or fused ring (i.e., ringswhich share an adjacent pair of atoms) group having in the ring(s) oneor more atoms, such as, for example, nitrogen, oxygen and sulfur and, inaddition, having a completely conjugated pi-electron system. Examples,without limitation, of heteroaryl groups include pyrrole, furan,thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrimidine,quinoline, isoquinoline and purine. The heteroaryl group may besubstituted or unsubstituted. Substituted heteroaryl may have one ormore substituents, whereby each substituent group can independently be,for example, hydroxyalkyl, trihaloalkyl, cycloalkyl, alkenyl, alkynyl,aryl, heteroaryl, heteroalicyclic, amine, halide, sulfonate, sulfoxide,phosphonate, hydroxy, alkoxy, aryloxy, thiohydroxy, thioalkoxy,thioaryloxy, cyano, nitro, azo, sulfonamide, C-carboxylate,O-carboxylate, N-thiocarbamate, O-thiocarbamate, urea, thiourea,O-carbamate, N-carbamate, C-amide, N-amide, guanyl, guanidine andhydrazine. The heteroaryl group can be an end group, as this phrase isdefined hereinabove, where it is attached to a single adjacent atom, ora linking group, as this phrase is defined hereinabove, connecting twoor more moieties at two or more positions thereof. Representativeexamples are pyridine, pyrrole, oxazole, indole, purine and the like.

The term “halide” and “halo” describes fluorine, chlorine, bromine oriodine.

The term “haloalkyl” describes an alkyl group as defined above, furthersubstituted by one or more halide.

The term “sulfate” describes a —O—S(═O)₂—OR′ end group, as this term isdefined hereinabove, or an —O—S(═O)₂—O— linking group, as these phrasesare defined hereinabove, where R′ is as defined hereinabove.

The term “thiosulfate” describes a —O—S(═S)(═O)—OR′ end group or a—O—S(═S)(═O)—O— linking group, as these phrases are defined hereinabove,where R′ is as defined hereinabove.

The term “sulfite” describes an —O—S(═O)—O—R′ end group or a —O—S(═O)—O—group linking group, as these phrases are defined hereinabove, where R′is as defined hereinabove.

The term “thiosulfite” describes a —O—S(═S)—O—R′ end group or an—O—S(═S)—O— group linking group, as these phrases are definedhereinabove, where R′ is as defined hereinabove.

The term “sulfinate” describes a —S(═O)—OR′ end group or an —S(═O)—O—group linking group, as these phrases are defined hereinabove, where R′is as defined hereinabove.

The term “sulfoxide” or “sulfinyl” describes a —S(═O)R′ end group or an—S(═O)— linking group, as these phrases are defined hereinabove, whereR′ is as defined hereinabove.

The term “sulfonate” describes a —S(═O)₂—R′ end group or an —S(═O)₂—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

The term “S-sulfonamide” describes a —S(═O)₂—NR′R″ end group or a—S(═O)₂—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein. The term “N-sulfonamide” describes anR′S(═O)₂—NR″— end group or a —S(═O)₂—NR′— linking group, as thesephrases are defined hereinabove, where R′ and R″ are as defined herein.

The term “disulfide” refers to a —S—SR′ end group or a —S—S— linkinggroup, as these phrases are defined hereinabove, where R′ is as definedherein.

The term “phosphonate” describes a —P(═O)(OR′)(OR″) end group or a—P(═O)(OR′)(O)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “thiophosphonate” describes a —P(═S)(OR′)(OR″) end group or a—P(═S)(OR′)(O)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphinyl” describes a —PR′R″ end group or a —PR′— linkinggroup, as these phrases are defined hereinabove, with R′ and R″ asdefined hereinabove.

The term “phosphine oxide” describes a —P(═O)(R′)(R″) end group or a—P(═O)(R′)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphine sulfide” describes a —P(═S)(R′)(R″) end group or a—P(═S)(R′)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “phosphite” describes an —O—PR′(═O)(OR″) end group or an—O—PH(═O)(O)— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “carbonyl” or “carbonate” as used herein, describes a —C(═O)—R′end group or a —C(═O)— linking group, as these phrases are definedhereinabove, with R′ as defined herein.

The term “thiocarbonyl” as used herein, describes a —C(═S)—R′ end groupor a —C(═S)— linking group, as these phrases are defined hereinabove,with R′ as defined herein.

The term “oxo” as used herein, describes a (═O) group, wherein an oxygenatom is linked by a double bond to the atom (e.g., carbon atom) at theindicated position.

The term “thiooxo” as used herein, describes a (═S) group, wherein asulfur atom is linked by a double bond to the atom (e.g., carbon atom)at the indicated position.

The term “oxime” describes a ═N—OH end group or a ═N—O— linking group,as these phrases are defined hereinabove.

The term “hydroxyl” describes a —OH group.

The term “alkoxy” describes both an —O-alkyl and an —O-cycloalkyl group,as defined herein.

The term “aryloxy” describes both an —O-aryl and an —O-heteroaryl group,as defined herein.

The term “thiohydroxy” describes a —SH group.

The term “thioalkoxy” describes both a —S-alkyl group, and a—S-cycloalkyl group, as defined herein.

The term “thioaryloxy” describes both a —S-aryl and a —S-heteroarylgroup, as defined herein.

The “hydroxyalkyl” is also referred to herein as “alcohol”, anddescribes an alkyl, as defined herein, substituted by a hydroxy group.

The term “cyano” describes a —C≡N group.

The term “isocyanate” describes an —N═C═O group.

The term “isothiocyanate” describes an —N═C═S group.

The term “nitro” describes an —NO₂ group.

The term “acyl halide” describes a —(C═O)R″″ group wherein R″″ ishalide, as defined hereinabove.

The term “azo” or “diazo” describes an —N═NR′ end group or an —N═N—linking group, as these phrases are defined hereinabove, with R′ asdefined hereinabove.

The term “peroxo” describes an —O—OR′ end group or an —O—O— linkinggroup, as these phrases are defined hereinabove, with R′ as definedhereinabove.

The term “carboxylate” as used herein encompasses C-carboxylate andO-carboxylate.

The term “C-carboxylate” describes a —C(═O)—OR′ end group or a —C(═O)—O—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

The term “O-carboxylate” describes a —OC(═O)R′ end group or a —OC(═O)—linking group, as these phrases are defined hereinabove, where R′ is asdefined herein.

A carboxylate can be linear or cyclic. When cyclic, R′ and the carbonatom are linked together to form a ring, in C-carboxylate, and thisgroup is also referred to as lactone. Alternatively, R′ and O are linkedtogether to form a ring in O-carboxylate. Cyclic carboxylates canfunction as a linking group, for example, when an atom in the formedring is linked to another group.

The term “thiocarboxylate” as used herein encompasses C-thiocarboxylateand O-thiocarboxylate.

The term “C-thiocarboxylate” describes a —C(═S)—OR′ end group or a—C(═S)—O— linking group, as these phrases are defined hereinabove, whereR′ is as defined herein.

The term “O-thiocarboxylate” describes a —OC(═S)R′ end group or a—OC(═S)— linking group, as these phrases are defined hereinabove, whereR′ is as defined herein.

A thiocarboxylate can be linear or cyclic. When cyclic, R′ and thecarbon atom are linked together to form a ring, in C-thiocarboxylate,and this group is also referred to as thiolactone. Alternatively, R′ andO are linked together to form a ring in O-thiocarboxylate. Cyclicthiocarboxylates can function as a linking group, for example, when anatom in the formed ring is linked to another group.

The term “carbamate” as used herein encompasses N-carbamate andO-carbamate.

The term “N-carbamate” describes an R″OC(═O)—NR′— end group or a—OC(═O)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “O-carbamate” describes an —OC(═O)—NR′R″ end group or an—OC(═O)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

A carbamate can be linear or cyclic. When cyclic, R′ and the carbon atomare linked together to form a ring, in O-carbamate. Alternatively, R′and O are linked together to form a ring in N-carbamate. Cycliccarbamates can function as a linking group, for example, when an atom inthe formed ring is linked to another group.

The term “carbamate” as used herein encompasses N-carbamate andO-carbamate.

The term “thiocarbamate” as used herein encompasses N-thiocarbamate andO-thiocarbamate.

The term “O-thiocarbamate” describes a —OC(═S)—NR′R″ end group or a—OC(═S)—NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “N-thiocarbamate” describes an R″OC(═S)NR′— end group or a—OC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

Thiocarbamates can be linear or cyclic, as described herein forcarbamates.

The term “dithiocarbamate” as used herein encompasses S-dithiocarbamateand N-dithiocarbamate.

The term “S-dithiocarbamate” describes a —SC(═S)—NR′R″ end group or a—SC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “N-dithiocarbamate” describes an R″SC(═S)NR′— end group or a—SC(═S)NR′— linking group, as these phrases are defined hereinabove,with R′ and R″ as defined herein.

The term “urea”, which is also referred to herein as “ureido”, describesa —NR′C(═O)—NR″R′″ end group or a —NR′C(═O)—NR″— linking group, as thesephrases are defined hereinabove, where R′ and R″ are as defined hereinand R′″ is as defined herein for R′ and R″.

The term “thiourea”, which is also referred to herein as “thioureido”,describes a —NR′—C(═S)—NR″R′″ end group or a —NR′—C(═S)—NR″— linkinggroup, with R′, R″ and R′″ as defined herein.

The term “amide” as used herein encompasses C-amide and N-amide.

The term “C-amide” describes a —C(═O)—NR′R″ end group or a —C(═O)—NR′—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

The term “N-amide” describes a R′C(═O)—NR″— end group or a R′C(═O)—N—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

An amide can be linear or cyclic. When cyclic, R′ and the carbon atomare linked together to form a ring, in C-amide, and this group is alsoreferred to as lactam. Cyclic amides can function as a linking group,for example, when an atom in the formed ring is linked to another group.

The term “guanyl” describes a R′R″NC(═N)— end group or a —R′NC(═N)—linking group, as these phrases are defined hereinabove, where R′ and R″are as defined herein.

The term “guanidine” describes a —R′NC(═N)—NR″R′″ end group or a—R′NC(═N)—NR″— linking group, as these phrases are defined hereinabove,where R′, R″ and R′″ are as defined herein.

The term “hydrazine” describes a —NR′—NR″R′″ end group or a —NR′—NR″—linking group, as these phrases are defined hereinabove, with R′, R″,and R′″ as defined herein.

As used herein, the term “hydrazide” describes a —C(═O)—NR′—NR″R′″ endgroup or a —C(═O)—NR′—NR″— linking group, as these phrases are definedhereinabove, where R′, R″ and R′″ are as defined herein.

As used herein, the term “thiohydrazide” describes a —C(═S)—NR′—NR″R′″end group or a —C(═S)—NR′—NR″— linking group, as these phrases aredefined hereinabove, where R′, R″ and R′″ are as defined herein.

As used herein, the term “alkylene glycol” describes a—O—[(CR′R″)_(z)—O]_(y)R′″ end group or a —O—[(CR′R″)_(z)—O]_(y)— linkinggroup, with R′, R″ and R′″ being as defined herein, and with z being aninteger of from 1 to 10, preferably, from 2 to 6, more preferably 2 or3, and y being an integer of 1 or more. Preferably R′ and R″ are bothhydrogen. When z is 2 and y is 1, this group is ethylene glycol. When zis 3 and y is 1, this group is propylene glycol. When y is 2-4, thealkylene glycol is referred to herein as oligo(alkylene glycol).

When y is greater than 4, the alkylene glycol is referred to herein aspoly(alkylene glycol). In some embodiments of the present invention, apoly(alkylene glycol) group or moiety can have from 10 to 200 repeatingalkylene glycol units, such that z is 10 to 200, preferably 10-100, morepreferably 10-50.

The term “silanol” describes a —Si(OH)R′R″ group, or —Si(OH)₂R′ group or—Si(OH)₃ group, with R′ and R″ as described herein.

The term “silyl” describes a —SiR′R″R′″ group, with R′, R″ and R′″ asdescribed herein.

As used herein, the term “urethane” or “urethane moiety” or “urethanegroup” describes a Rx-O—C(═O)—NR′R″ end group or a -Rx-O—C(═O)—NR′—linking group, with R′ and R″ being as defined herein, and Rx being analkyl, cycloalkyl, aryl, alkylene glycol or any combination thereof.Preferably R′ and R″ are both hydrogen.

The term “polyurethane” or “oligourethane” describes a moiety thatcomprises at least one urethane group as described herein in therepeating backbone units thereof, or at least one urethane bond,—O—C(═O)—NR′—, in the repeating backbone units thereof.

It is expected that during the life of a patent maturing from thisapplication many relevant curable materials featuring properties (e.g.,Tg when hardened) as described herein, will be developed, and the scopeof the respective curable materials is intended to include all such newmaterials a priori.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in a nonlimiting fashion.

Materials and Methods

3D Inkjet printing of shelled objects was performed using Objet C2, C3Systems, in a DM mode (e.g., a DM mode referred to as DM 5160 or 5130),according to the method described in U.S. Patent Application havingPublication No. 2013/0040091. Generally, all printed objects werecomprised of a core made of the first formulation (RF, Part A) and thesecond formulation (DLM, Part B), at a ratio as indicated, and one ormore shells, wherein, optionally, one shell comprises the firstformulation, and optionally, another shell comprises the secondformulation, unless otherwise indicated.

HDT measurements were performed on Ceast vicat/HDT instrument accordingto ASTM D-648-06.

Print deformations (curling) were quantitatively assessed using a230×10×10 mm printed bar. Upon printing, the bar was left within theprinting system, in a closed cabinet, for 1 hour, and was thereafterstored at room temperature for 24 hours. The bar was then placed on aflat plane (flat table) and curling was measured by putting weight onone side of the bar and measuring the height of the bar edge from theplane in mm. For this study an elevation of 4 mm or less was consideredas acceptable for most mainstream applications.

Tray temperature was measured directly by using Thermocouple connectedto data logger apparatus.

Measurements of other properties were performed according to standardprotocols, unless otherwise indicated.

All reagents and materials composing the tested formulations wereobtained from known vendors.

Example 1 Modeling Material Formulations and Printed Objects MadeTherefrom

The present inventors have searched for formulations which can be usedin AM of shelled objects, yet, would exhibit improved properties such asimproved HDT and improved Impact resistance, and reduced curling.

The present inventors have recognized that while curling can be reducedby applying heat to the dispensed layers, such heat can only be appliedwhen the formulations feature, when hardened, HDT above the heattemperature, yet, that elevated HDT of a hardened formulation typicallyresults in reduced Impact resistance.

The present inventors have therefore thought for formulations whichprovide hardened material with both high HDT and high Impact resistance,when used in AM of shelled objects, and which allow heating thedispensed layers so as to reduce or avoid curling, without causingdeformation of the printed layers upon said heating.

As described in detail in U.S. Patent Application having Publication No.2013/0040091, solid freeform fabrication (SFF) of shelled objects isperformed using two formulations: a first formulation, Part A, referredto also as RF (reinforcer); and a second formulation, Part B, referredto also as DLM.

The present inventors have now designed and successfully practiced in AMof shelled objects the following formulations.

A First Formulation:

The first formulation (Part A, RF) was designed so as to provide, whenhardened, a rigid material characterized by high HDT (e.g., higher than90° C., or higher than 100° C.).

The present inventors have uncovered that the first formulation shouldinclude a curable methacrylic monomer which features, when hardened, Tghigher than 150° C. In studies described in further detail hereinafter,the present inventors have characterized a desired curing rate of such amonomer, namely, a curing rate lower than 5,000 mW/minute or lower than4,400 mW/minute (for a curing rate determined as described hereinafter).

The present inventors have further determined that an amount of such amonomer should be at least 35% by weight of the total weight of thefirst formulation.

The first formulation, according to embodiments of the presentinvention, may further include:

at least one curable acrylic monomer characterized, when hardened, by Tgof at least 85° C.;

at least one curable (meth)acrylic oligomer, characterized, whenhardened, by Tg of at least 50° C.; and

optionally, at least one curable (meth)acrylic monomer characterized,when hardened, by Tg lower than 0° C., or lower than −20° C.,

and further optionally, a photoinitiator, a surfactant, a dispersingagent and/or an inhibitor.

Table 1 below presents exemplary materials suitable for inclusion in thefirst (Part A, RF) formulation, according to some embodiments of thepresent invention:

TABLE 1 Percentage Component Exemplary Materials (%) Curable acrylicmonomer, ACMO (CAS: 5117-12-4 ) 10-40 characterized, when (Tg = 88° C.)hardened, by Tg > 85° C. IBOA (CAS:5888-33-5) (Tg = 95° C.) SR 833S(CAS: 42594-17-2) (Tg = 185° C.) SR454 ethoxylated (3) TMPTA (CAS:28961-43-5) (Tg = 103° C.) SR508 (CAS 57472-68-1) (Tg = 104° C.) SR368(CAS: 40220-08-4) (Tg = 272° C.) Curable Methacrylic SR 834 (CAS:43048-08-4) 35-50 monomer, characterized, SR-423D (CAS: 7534-94-3) whenhardened, by Tg > 150° C. (Meth) acrylic Oligomer, BR-441 (Di functionalAliphatic 10-40 characterized, when polyester urethane Acrylate)hardened, by Tg > 50° C. (Tg = 71° C.) PH 6019 (Trifunctional Aliphaticurethane acrylate) (Tg = 51° C.) Eb3703 amine modified epoxy Diacrylate(Tg = 57° C.) (Meth)acrylic flexible SR 9036 (Ethoxylated (30) bisphenolA  5-30 monomer, Having low dimethacrylate) (CAS: 41637-38-1) Tg < −20°C (Tg = −43° C.) SR415 (Ethoxylated (20) Trimethylol propanetriacrylate) (CAS: 28961-43-5) (Tg = −40° C.) SR 9035 (Ethoxylated (15)Trimethylol propane triacrylate (Tg = −30° C.) SR610 (Poly(ethyleneglycol) (600) diacrylate) (Tg = −40° C.) Photoinitiator BAPO type (BisAcyl Phosphine Oxide) 0.5-5  Alpha Hydroxy ketone MAPO(Monoacylphosphine oxides) Surfactant BYK Type (PDMS derivatives)0.01-1   Dispersing agent BYK Type (PDMS derivatives) 0.01-1   InhibitorMEHQ 0.1-1  Genorad Type Inorganic Pigment Nano scale Titanium Oxide0.1-0.3 Nano scale Zirconium Oxide Nano Silica Organic pigment Nanoscale Carbon black  0.1-0.15

Photo DSC Measurements:

Photo DSC measurements were performed on Mettler Toledo, withLightningcure™: Curable type: Alumina curable; Sample size: 10-15 mg.

Tests were performed while maintaining a sample for 2 minutes at 60° C.,thereafter subject it to polymerization with a single flash of 0.1second duration and then maintaining it for additional 1 minute at 60°C. Sampling interval of 0.1 second was used for reading the measurementdata.

The following materials and formulations were tested:

RGD531, DR-71-black2 (also referred to herein as RF71), Modified DR-71in which SR834 is replaced by SR833 and SR844.

Table 2 below presents the chemical composition of RGD531.

Table 3 below presents the chemical composition GR-71-black2 (RF71).

TABLE 2 Material Wt. percentage (%) Mono functional Acrylic monomer10-30 Tg > 85° C. Methacrylic monomer, Tg > 150° C.  5-20 Polyesterbased Urethane Acrylate  5-20 (Meth)acrylic Oilgomer, Tg > 50° C. 10-20Acrylic multi functional monomer 20-30 Tg > 85° C. Inhibitor 0.1-0.3Photoinitiator 0.5-3  Surfactant 0.01-1   Epoxy Acrylate 1-5 Dispersingagent 0.01-0.05 Inorganic pigment nano scale 0.2-0.6

TABLE 3 Material Wt. percentage (%) Mono functional Acrylic monomer10-30 Tg > 85° C. Multi functional Acrylic monomer 10-20 Tg > 85° C. SR834 >35% (Meth)acrylic Oligomer, Tg > 50° C. 10-20 (Meth) acrylicflexible monomer,  5-15 Having low Tg < −20° C Inhibitor 0.1-0.3Photoinitiator 2-5 Surfactant 0.01-1   Dispersant 0.01-1   Organicpigment  0.5-0.15

Table 4 below presents the data obtained in the Photo DSC measurementsfor each of the tested formulations and materials. The curing rate isdetermined by the slope of the curves obtained for the Mw (MiliWatt) asa function of time, for each sample.

TABLE 4 Enthalpy HDT/Tg Curing Rate Formulation/Material (J/gram) (° C.)(Slope; mW/minute) RGD531 97.9 HDT: 90-100 4400 DR-71 52.4 HDT: 140-1502480 Modified DR-71 142.1 HDT: 115 9980 SR833 331.7 Tg: 185 24000 SR83463.4 Tg: >200 3300

As shown in Table 4, at lower curing rates, higher HDT values areobtained. Such high HDT values are obtained only when SR-834 is in anamount of 35% wt. or higher. In the formulations where SR-834 is absentor is replaced by a material that is known to cure faster, such high HDTvalues were not obtained.

The Second Formulation:

The second formulation (Part B, DLM) was designed so as to provide, whenhardened, a material which is less rigid than the material obtained froma hardened first formulation, and which is characterized by hightoughness (e.g., Izod notch Impact of about 52-58 J/m), and HDT lowerthan the first hardened formulation (e.g., HDT of about 40-41° C.).

In studies described in further detail hereinafter, the presentinventors have uncovered that the second formulation should include atrifunctional curable (meth)acrylic (acrylic or methacrylic) monomerwhich comprises (e.g., at least 5) ethoxylated groups (also referred toherein as ethoxylated (meth)acrylic monomer, or simply as ethoxylatedmonomer or material). The present inventors have further determined thatimproved performance is obtained when the ethoxylated trifunctionalmonomer is characterized by a viscosity at room temperature lower than1000 centipoises; and/or a molecular weight of at least 500 grams/mol.The ethoxylated material features, when hardened, Tg lower than −20° C.

The present inventors have further determined that an amount of such atrifunctional monomer should be at least 5% by weight of the totalweight of the first formulation.

The second formulation, according to embodiments of the presentinvention, may further include:

at least one curable (meth)acrylic monomer characterized, when hardened,by Tg of at least70° C.; and

at least one curable (meth)acrylic oligomer characterized, whenhardened, by Tg of at least 10° C.,

and optionally, a photoinitiator, a surfactant, a dispersing agentand/or an inhibitor.

Table 5 below presents exemplary materials suitable for inclusion in thesecond (Part B, DLM) formulation, according to some embodiments of thepresent invention:

TABLE 5 Percentage Material Examples (%) Curable (meth) acrylic ACMO(CAS: 5117-12-4)  10-50 monomer, characterized, (Tg = 88° C.) whenhardened, by Tg > 85° C. IBOA (CAS: 5888-33-5) (Tg = 95° C.) SR 833S(CAS: 42594-17-2) (Tg = 185° C.) SR454 ethoxylated (3) TMPTA (CAS28961-43-5) (Tg = 103° C.) SR5O8 (CAS 57472-68-1) (Tg = 104° C.) SR368(CAS 40220-08-4) (Tg = 272° C.) SR423 (CAS 7534-94-3) (Tg = 110° C.)Curable (meth)acrylic CN-991 (Aliphatic polyester based  10-50 oligomercharacterized, Urethane diacrylate) when hardened, by (Tg = 40° C.) Tg >10° C. PH 6019 Aliphatic Urethane TriAcrylate (Tg = 51° C.) Eb3708(Modified bisphenol-A epoxy diacrylate) (Tg = 21° C.) Curableethoxylated SR 9036 (Ethoxylated (30) bisphenol A   5-40 trifunctional(meth) acrylic dimethacrylate) (CAS 41637-38-1) monomer, characterized,(Tg = −43° C.) when hardened, by SR415 (Ethoxylated (20) Trimethylol Tg< −20° C. propane triacrylate) (CAS 28961-43-5, Tg = −40° C.) SR 9035(Ethoxylated (15) Trimethylol propane triacrylate) (Tg = −30° C.) SR610(Poly(ethylene glycol) (600) diacrylate) (Tg = −40° C.) * Other examplesare shown in Table 6 below Photoinitiator BAPO type (Bis Acyl PhosphineOxide) 0.5-5 Alpha Hydroxy ketone MAPO (Monoacylphosphine oxides)Surfactant BYK Type (PDMS derivatives) 0.01-1  Dispersing agent BYK Type(PDMS derivatives) 0.01-1  Inhibitor MEHQ 0.1-1 Genorad Type

The present inventors have uncovered that ethoxylated monomers increasethe Impact values of the hardened formulation, and can further provideHDT of 40° C. or higher.

Table 6A below presents exemplary ethoxylated materials, and theirproperties, which are suitable for inclusion in the second formulation(Part B). The Impact and HDT provided by these formulations arepresented in Table 6C below.

Table 6B below presents exemplary materials featuring low Tg which werefound in the above-described experiments, as failing to provide thedesired properties.

TABLE 6A Number of MW Ethoxylated Viscosity Material (gram/mol) groups(Cp at 25° C.) SR-9036 Ethoxylated (30) 2156 30 610 bisphenol Adimethacrylate SR-415 Ethoxylated (20) 1176 20 225 TrimethylolpropaneTriacrylate SR430 Ethoxylated 18 1249 18 825 Tristyrylphenol acrylate(RSP(18EO)A) SR9035 Ethoxylated 15 956 15 177 TrimethylolpropaneTriacrylate SR567P Ethoxylated 25 C22 1494 25 250 methacrylate SR480Ethoxylated 10 808 10 410 bisphenol A DMA SR499 Ethoxylated (6) 554 6 92Trimethylolpropane Triacrylate SR-610 PEG 600 diacrylate 726 13 100

TABLE 6B Izod Impact HDT Material (J/m) (° C.) CN-131 Mono functionalAromatic 14.1 <40 Epoxy Acrylate Genomer Aliphatic Urethane 15 <404188/EHA Acrylate CN-990 Siliconized Urethane 29.07 41 Acrylate OligomerSR 395 Iso Decyl Acrylate <15 <40 SR-256 2(2-Ethoxylated) 21.6 <40EthylAcrylate (EOEOEA) SR-495 Capro lactone Acrylate 40 40 SR-395 IsoDecyl Acrylate <15 <40

The present inventors have further tested the properties obtained whenusing two Part B formulations deferring by the type of ethoxylatedmonomer. One formulation is RGD515, which comprises, as an ethoxylatedmonomer, SR9036 (see, Table 6A), and in the other, SR9036 was replacedby SR415 (see, Table 6A).

The properties provided by these formulations are presented in Table 6C.

TABLE 6C Impact, J/m HDT, ° C. Formulation with SR9036 80 40.9Formulation with SR415 50 42

As can be seen from Tables 6A-6C, low Tg curable monomers should featureat least 3, preferably at least 5 ethoxylated groups, a viscosity rangeof 50-1000 centipoises (Cp) at room temperature; and MW range of500-3000 grams/mol.

While formulations comprising a difunctional ethoxylated monomer featurehigher Impact, formulations comprising a trifunctional ethoxylatedmonomer feature higher HDT, improved dimensional stability, and higherElastic modulus, and thus are advantageous.

In some embodiments, a concentration of this material in the secondformulation is in a range of 10-50% by weight.

Elastic Moduli Ratio:

In some embodiments of the present invention, the first and the secondmodeling formulations are selected according to their characteristicelastic moduli. Computer simulations have been conducted in order todetermine a preferred ratio between the elastic moduli of the twomodeling formulations. The computer simulations were performed forvarious combinations in which the first modeling formulation is aformulation that is commercially available under the trade name RGD531,and having an elastic modulus of 3000 MPa. Seven types of formulationswere tested as the second formulations. These are referred to asSoft-30, Soft-16, RGD515, M-1, M-2, M-3, and M-4.

The computer simulations included analysis of stress distributionresulting from a crack in the second modeling formulation. The resultsof the simulations are provided in Table 7 and FIGS. 1A-1G. In FIGS.1A-1G, the lower layer corresponds to the first modeling formulation(RGD531, in the present Example), and the upper layer corresponds to therespective second modeling formulation (Soft-30, Soft-16, RGD515, M-1,M-2, M-3, and M-4, respectively).

TABLE 7 Second Young's modulus Formulation [MPa] Max stress value andlocation Soft-30 90 400 MPa in the first modeling formulation Soft-16550 283 MPa in the first modeling formulation RGD515 1000 250 MPa in thefirst modeling formulation 250 MPa at the interface between the twoformulations, under the crack M-1 1330 main stress of 250 MPa at thebottom of the crack and at the interface between the two formulations,under the crack 200 MPa in the first modeling formulation M-2 1600 mainstress of 257 MPa at the bottom of the crack and at the interfacebetween the two formulations under the crack 257 MPa in the firstmodeling formulation M-3 1700 main stress of 269 MPa at the bottom ofthe crack 220 MPa at the interface between the two formulations, underthe crack M-4 1800 main stress of 285 MPa at the bottom of the 220 MPaat the interface between the two formulations, under the crack

Table 7 demonstrates that there is a ratio between the elastic modulifor which the distribution of stress is optimal. In the present example,the optimal distribution of stress is achieved when the elastic modulusof the second modeling formulation is from about 1000 to about 1330,corresponding to a ratio between the elastic moduli of from 2.7 to 3.0.

In a preferred embodiment of the invention, the ratio between theelastic modulus of the first formulation and the elastic modulus of thesecond formulation is from about 2.7 to about 2.9, more preferably fromabout 2.75 to about 2.85. This preferred ratio is based on theobservation by the present Inventors that when the first modelingformulation has an elastic modulus of 3000 MPa, the optimal distributionis obtainable for a second modeling formulation having an elasticmodulus which is higher than the elastic modulus of RGD515 (1000 MPa)and lower than the elastic modulus of M-1 (1330 MPa).

It was found by the present Inventors that the preferred ratios areobtainable, for example, when the first modeling formulation isDR-71-Black2, also referred to herein as DR-71 or RF71, or is aformulation referred to herein as DR-71* or RF71*, and the secondmodeling formulation is Di-69-1, which is also referred to herein asDI-69.

Example 2 The Heating Element

Exemplary first and second formulations according to the presentembodiments were used in a method as described herein in printing anobject shaped as a bar having dimensions of 230×10×10 mm, while heatingthe receiving medium (tray) at various temperatures (without applying IRlamp radiation). The effect of tray temperature on the HDT and thecurling (with no pedestal) of the printed object is presented in Table 8below.

TABLE 8 Tray HDT Temperature Out of the printer Curling (° C.) (° C.)(mm) 38.9 65.7 ± 1.6 2.7 49.3 71.1 ± 1.6 1.7 60.3 75.6 ± 1.1 0.5 72.075.5 ± 0.7 −1.2 87.9 93.0 ± 1.0 −3.0 99.6 97.5 −3.5

As can be seen, at 60° C., no curling was observed.

The effect of heating the tray to 60° C. while further operating aceramic lamp is presented in Table 9.

TABLE 9 Ceramic Lamp HDT Voltage Out of the printer Curling (Volts) (°C.) (mm) 0 75.6 ± 1.1 0.5 160 78.9 ± 1.3 0.8 180 82.6 ± 1.8 1.1 215 91.6± 1.0 2.0

In additional assays, the effect of thermal post treatment, at 150° C.,for two hours, on the HDT of the printed object was tested, with andwithout application of heat to the dispensed layers.

HDT of the printed object was measured for “Cold Printer”, where noheating was applied to the dispensed layers; and for “HOT Printer”,where tray was heated at 60° C. and a Ceramic IR Lamp was operated at180 Volts. The obtained data is presented in Table 10.

TABLE 10 HDT HDT After Thermal Ceramic Lamp Out of the printer posttreatment Power (Volts) (° C.) (150° C., 2 Hours) Cold printer 88.5 ±4.4 146.8 ± 1.8 Hot printer 101-112 146-151

FIG. 4 is a graph that shows the temperature of the printed object thatensures curling of less than 3 mm, as a function of the final HDT of theprinted object, for four different formulation or combination offormulations. As shown, the temperature of the printed object thatensures reduced curling increases with the final HDT of the object. Forexample, for printing ABS DM having final HDT of about 90° C., it issufficient to heat the printed object to 43° C. For printing ABS DMhaving final HDT of 165° C., it is preferred to heat the printed objectto about 67° C.

Example 3

The Printed Object

Effect of RF Concentration in the Core:

The effect of the concentration of the first formulation (Part A, RF) inthe core region on the HDT of the core and of the final object wastested.

In exemplary measurements, a Part A formulation referred to herein asRF71 was used in combination with a Part B formulation, referred toherein as DI-69.

In additional exemplary measurements, a Part A formulation referred toherein as RF4w was used in combination with a DI-69 Part B formulation.

Table 11 below presents the chemical composition of RF4w.

TABLE 11 Material Wt. percentage (%) Mono functional Acrylic monomer10-30 Tg > 85° C. Multi functional Acrylic monomer 10-30 Tg > 85° C.SR834 >35% Polyester Urethane Acrylate  5-15 (Meth) acrylic flexiblemonomer. 10-30 Having low Tg < −20° C Inhibitor 0.1-0.3 Photoinitiator1-5 Surfactant 0.01-1   Dispersant 0.01-1   Inorganic pigment 0.5-1 

Samples having a thickness of 6.35 mm were printed as follows:

DM-ABS or DABS: Full DM 5160 Structure

DM-ABS PC or DABS PC: same with thermal post curing

RND: Only the Core structure, Random DM

RND PC: Same with thermal post curing

FIGS. 2A-2B present the effect of various concentrations of RF71 in thecore on the HDT of the various printed objects, 6.35 mm in thickness(FIG. 2A), and the effect of various concentrations of RF4w in the coreon the HDT of the various printed objects, 6.35 mm in thickness (FIG.2B).

As can be seen in FIGS. 2A-2B, increasing the amount of the Part Aformulation in the core region increases the HDT of the core. However,in the ABS DM mode, when the core is surrounded by a shell, the overallHDT is generally the same, irrespectively of the percentage of the firstmodeling formulation in the core. The same trend is observed for bothtested formulations, and for both samples, indicating that the HDT ofthe object is not affected by the relative amount of Part A in the core.

Further the effect of the concentration of the first formulation (PartA, RF) in the core region on the Impact resistance (Izod) of the finalobject was tested.

In exemplary measurements, a Part A formulation referred to herein asRF71 was used in combination with a Part B formulation, referred toherein as DI-69.

In additional exemplary measurements, a Part A formulation referred toherein as RF4w was used in combination with a DI-69 Part B formulation.

FIGS. 3A-3B present the effect of various concentrations of RF71 or RF4w(FIG. 3A) and of a previously described formulation, RG535 (FIG. 3B) inthe core, on the Impact resistance of the various printed objects, 6.35mm in thickness. As shown therein, a pronounced improvement in Impactresistance is observed when using RF71, compared to previously describedformulations, already at 25% by weight of this formulation in the core.It is shown that in the ABS DM mode, when the core is surrounded by ashell, the Impact resistance is generally the same, irrespectively ofthe percentage of the first modeling formulation in the core. The sametrend is observed for both tested formulations, indicating that theImpact resistance of the object is not affected by the relative amountof Part A in the core (but rather is mainly dependent on the compositionof the shells).

The Effect of a DLM Concentration in the Core:

The effect of the concentration of a second modeling materialformulation (or of the RF/DLM ratio) in the core, in a layered structurefeaturing a DM-ABS structure as described herein, on the Loss Modulus,Tg, Storage Modulus and Elastic Modulus was tested at varioustemperatures, using a second modeling formulation (DLM) referred toherein as Di-69, in combination with a first modeling material (RF)referred to herein as RF4w, or a first modeling material (RF) referredto herein as RF71.

The tested sample was a rectangle 3 mm overall thickness, overall lengthof 30 mm, with 17 mm between the two fixtures (span), and 13 mm width.Test was made in single cantilever mode, fabricated by athree-dimensional inkjet printing system, using a heated tray, at atemperature from ambient to 150° C.

The thickness of each layer was 32 μm. Each layer was printed by randominterlacing in the core and/or the stitching layer, if present, of therespective first modeling formulation and a second modeling formulationto form a digital material.

Measurements were performed using a digital mechanical analysis (DMA)system, model Q800, available from TA Instruments, Inc., of New Castle,Del. The DMA system was operated in a single cantilever mode,oscillation mode, temperature ramp, frequency of 1 Hz and heating rateof 3° C./min.

Dynamic mechanical analysis was performed for the fabricated layeredcores. The dynamic mechanical analysis provided the trigonometricfunction tan(δ), where δ is the phase between the stress σ and thestrain ε. The function tan(δ) is a viscoelastic property that iscorrelated with the damping of the respective formulation, or, morespecifically, the ability of the respective formulation to dissipatemechanical energy by converting the mechanical energy into heat.

The results are presented in Tables 12 and 13 and FIGS. 12A and 12B, forvarious concentrations of Di-69 when used with RF4w as the RFformulation.

TABLE 12 Di-69 (% wt.) E′ @ 100 C. (MPa) 75 76.1 50 264 40 372 30 542 10910

TABLE 13 Di-69 (% wt.) Tg (G″) 75 62.9 50 72.6 40 78 20 84.5 10 94.6

The same experiments were conducted using RF71, as described herein, asthe RF formulation. The results are presented in Tables 14 and 15 andFIGS. 13A and 13B, for various concentrations of Di-69 when used withRF71 as the RF formulation.

TABLE 14 Di-69 (% wt.) Tg (G″) 75 67.9 50 84.2 40 91.8 25 107.5 10 118.60 124.1

TABLE 15 Di-69 (% wt.) E′ @ 100 C. (MPa) 75 139 50 554 40 831 25 1174 101350 0 1600

These data indicate that the concentration of the first and/or secondformulation in the core can be manipulated in accordance with desiredmechanical properties of the obtained object at an environmentaltemperature of its intended use.

The Effect of the RF/DLM Weight Ratio in the Core:

In further experiments conducted, exemplary RF formulations according tothe present embodiments, RF71 and RF4w, and the previously described RFformulation RF 535, were tested in combination with a DLM formulationreferred to herein as Di69, at various ratios, for the purpose oftesting the effect of the ratio on the mechanical properties of anobject made therefrom. DMA measurements were performed as describedhereinabove, using a 3D-inkjet printed sample as described hereinabove.

The total calculated Tg values for these three types of first modelingformulation (obtained by sum of the individual Tg values of therespective components, weighted by the respective weight percentage, ofeach modeling formulation) are 127° C., 107° C. and 146° C., for RF 4w,RF 535 and RF 71, respectively.

The data obtained for the Storage Modulus, at various temperatures, ofRF 4w, RF 535 and RF 71 at a 50:50 (1:1) weight ratio is presented inTable 16 and FIG. 14A, and of RF 4w, RF 535 and RF 71 at a 25:75 (1:3)weight ratio is presented in Table 17 and FIG. 14B.

TABLE 16 RF E′ @ 100 C. (MPa) 535 85 4w 265  71 560

TABLE 17 RF E′ @ 100 C. (MPa) 535 26 4w 76  71 140

The Effect of Post Curing:

In further studies conducted, the effect of post-curing on themechanical properties of the object, at various concentrations of eachof the above-mentioned RF formulation was tested. For RF4w, it was foundthat post-curing results in an increase of the storage modulus up to 90°C., while at higher temperatures it has no substantial effect (data notshown). Post curing decreases the tan(o), whereby a more substantialdecrease is seen at higher RF concentrations (data not shown). Attemperatures higher than 100° C., lower tan(δ) and similar storagemodulus are obtained for samples subjected or not to post-curing.

FIGS. 15A-15B show tan(o) as a function of the temperature, RF 4w, RF535 and RF 71, at a 25/75 weight ratio between the first and secondmodeling formulation (FIG. 15A) and at a 50/50 weight ratio between thefirst and second modeling formulation (FIG. 15B). As shown therein, forall three types of the first modeling formulation, the location andwidth of the peak of tan(o) as a function of the temperature varysmoothly and monotonically when the weight percentage of the firstmodeling formulation is increased. Importantly, it can be seen that afirst modeling material formulation according to the presentembodiments, provides higher damping values is therefore usable whenhigh damping is required for objects that are intended for use in anenvironmental temperature that is higher than 100° C., or higher than110° C., or higher than 120° C., or higher.

The RF Composition:

The present inventors have further uncovered that when an additional(meth)acrylic flexible monomer having low Tg<−20° C., is added to afirst (RF) formulation as described herein, the properties of thefabricated object can be further manipulated.

More specifically, the present inventors have uncovered that while suchan addition of a flexible monomer may lead to a decrease in the HDT ofthe fabricated object (yet in HDT values as described herein, of atleast 80 or at least 100° C.), properties such as elongation at breakand toughness (brittleness) are improved, as described in further detailhereinunder.

The additional flexible monomer can be, for example, an ethoxylatedmonomer as described herein. The additional flexible monomer can be thesame flexible (e.g., ethoxylated) monomer already included in a part Aformulation as described herein, or, preferably, a different flexiblemonomer. An exemplary such additional flexible monomer is apoly(alkylene glycol) diacrylate, such as a poly(ethylene glycol)diacrylate, which features at least 5, preferably at least 10, e.g.,from 10 to 15, alkylene glycol groups. An exemplary such additionalflexible monomer is SR-610.

The weight ratio between a part A formulation as described herein andthe additional flexible monomer can be in a range of from 70:30 to 95:5,or from 80:20 to 95:5, or from 85:15 to 95:5, or is 90:10.

Table 18 below presents a chemical composition of an exemplary RFformulation to which an additional flexible monomer was added. This RFformulation comprises 90% by weight of a formulation referred to hereinas RF71, to which 10% by weight of SR-610, as an exemplary additionalflexible monomer, was added, and which is referred to herein as RF71*.

TABLE 18 Material Wt. percentage (%) Mono functional Acrylic monomer9-27 Tg > 85° C. Multi functional Acrylic monomer 9-18 Tg > 85° C. SR834 ≥32% (Meth)acrylic Oligomer, 9-18 Tg > 50° C. (Meth)acrylic flexiblemonomer, 4.5-13.5 Having low Tg < −20° C Inhibitor 0.09-0.3 Photoinitiator 1.8-4.5  Surfactant 0.01-1    Dispersant 0.01-1   Organic pigment 0.5-0.15 An additional (Meth)acrylic 5-15 flexiblemonomer, Having low Tg < −20° C

Exemplary objects were prepared using a part A formulation referred toherein as RF71, and using a part A formulation referred to herein asRF71*. Objects were printed in ABS DM 5160 and 5161 modes, using DI-69as the Part B (DLM) formulation. Objects printed either is a glossy ormatte mode, with a support material marketed as SUP 705 used in a mattemode.

As used herein throughout, the terms “glossy” and “matte” mode refer tothe surface appearance of the printed model, after being subjected topost-curing (PC), or post-treatment (PT), as described herein.

In a matte mode, a support formulation is deposited concurrentlyimmediately adjacent to the model. Due to a tendency for the supportmaterial and the modeling material to mix at the interface between them,the resulted object features a matte appearance of the model surfaceafter support removal.

In a glossy mode, no support formulation is deposited concurrentlyimmediately adjacent to the model, the model surface appears glossy.

Properties such as HDT and Elongation at break were determined forobjects fabricated by glossy and matte modes, with and withoutpost-treatment, according to the respective ASTM.

Measurements for determining properties reflecting the toughness(brittleness) of the fabricated objects were designed by the presentinventors as follows:

Drop Test:

Glass-shaped objects were printed, featuring the following dimensions:Length—43 mm; Diameter—43 mm; and Wall Thickness—3 mm.

The glass-shaped objects were inserted into a round-shaped apparatusfeaturing at its center a void for holding the glass-shaped object.Pipes having a diameter slightly higher than 43 mm, and increasingheights, were placed above the glass-shaped object, and a metal weightof 352 grams was thrown through each pipe. The minimal height requiredto brake the glass by the falling metal weight was recorded.

Snap Test:

T-shape objects were printed, having the following dimensions: Length—40mm; Width—20 mm; Snap thickness—2.5 and 4.0 mm.

The Snap objects were placed in a Snap device of a LLOYD 5K (Load cell5K N) instrument, operated at a speed of 1 mm/minute, and the deflectionrequired to break the snap part was recorded.

Impact test (Z Direction):

Bar objects were printed in Zx orientation, having the followingdimensions: Length—65 mm; Width—12.7 mm; and Thickness—3 mm.

Impact resistance (Linear Resilience (J/m)) was recorded for each barusing a CHEAST Impact tester (non-notch).

Tables 19A and 19B present the data obtained for objects printed in aglossy mode (Table 19A) and a matte mode (Table 19B). Data is presentedfor objects subjected to post-treatment (PT), unless otherwiseindicated. As shown, toughness and elongation of objects printed usingRF71* are improved compared to RF71, while HDT slightly decreases.

TABLE 19A Snap Xz Snap Xz Impact HDT HDT Elongation Drop 2.5 mm 4.0 mmZx (No PT) (° C.) (PT) (° C.) (%) test (cm) (mm) (mm) (J/m) RF71 108 ± 5154 ± 1  8-11 55-65 2.12 ± 0.16 1.86 ± 0.03 698 ± 62 RF71* 85-90 118-12514-19 180-200 4.4 ± 0.9 2.5 ± 0.2 855 ± 17

TABLE 19B Drop test Snap Xz Snap Xz Impact Zx (cm) 2.5 mm (mm) 4.0 mm(mm) (J/m) RF71 35-45 1.89 ± 0.17 2.15 ± 0.27 19.09 ± 1.39 RF71* 55-653.1 ± 0.8 2.0 ± 0.3 23.2 ± 1.4

The Printed Objects:

Table 20 below presents various properties of printed objects preparedaccording to some embodiments of the present invention.

Properties were determined according to the indicated ASTM, and comparedto Stratasys product FDM Polycarbonate.

Object 1 refers to an ABS-DM object printed using RF71 and DI-69,according to the present embodiments, and Object 2 refers to an ABS-DMobject printed using RF71* and DI-69, according to the presentembodiments.

TABLE 20 Other FDM Poly printed Property ASTM carbonate Object 1 Object2 objects Tensile Strength D-638- 57 57-62 52-60 55-60 (MPa) 03 Tensilemodulus D-638- 1.944 2,600-3,000 2,000-2,500 2,600-3,000 (MPa) 04Elongation to D-638- 4.8      8-11 (PT)    14-19 (PT) 25-40 Break (%) 05Elongation to Yield D-638- 2.2 5-7 5-7 5-7 (%) 05 Flexural strengthD-790- 89 59-68 55-75 65-75 (MPa) 03 Flexural modulus D-790- 2,0061.562-1.766 1.400-2,100 1,700-2.200 (MPa) 04 HDT 0.45 MPa Out D-648- 88-107  90-100 58-68 of printer (° C. ) 06 HDT 0.45 MPa D-648- 138146-151 118-128 92-95 After Thermal PT 06 (° C.) Izod Noched D-256- 73 90-110  80-110 65-80 Impact (J/m) 06

As can be seen in Table 20, an object prepared according to the presentembodiments exhibits HDT and Impact resistance superior to polycarbonateand to objects made of previously disclosed formulations, while notcompromising other properties.

Example 4 Controlling Properties of a Fabricated Object

The data presented herein (see, for example, Example 3) demonstrate thatthe first formulation, and/or the relative amount of the first andsecond formulations can be selected to enhance or reduce mechanicalproperties of the fabricated object. For example, selection of the firstformulation and/or ratio that enhance the damping at high temperaturesprovides an object that is capable of dissipating energy, and that isless sensitive to internal stresses and crack propagation, when used ina high temperature environment. This applies to any first and secondformulations, including, but not limited to, the first and secondformulations of the present embodiments.

Accordingly, properties of a fabricated object can be controlled byjudicious selection of the modeling formulations and/or ratio between afirst and a second modeling formulation in the various regions of theobject, particularly the core region, if a core-shell structure isfabricated. For example, a predetermined damping range of the core, theshell or the entire object can be obtained by selecting an amount of afirst modeling formulation in the core, and/or by selecting one or moreparameters characterizing the first formulation. The damping range canbe expressed, for example, using the phase lag δ between the stress andthe strain of the core, the shell or the entire object, for a particulartemperature range.

In some of any of the embodiments described herein, the first and secondformulations are characterized by, feature, or selected so as tofeature, when hardened, the tangent of the phase lag between the stressand the strain of the respective structure (core, shell or the entireobject), which is at least 0.25 at a temperature range of from about 70°C. to about 90° C., or at least 0.20 at a temperature range of fromabout 90° C. to about 110° C., or at least 0.15 at a temperature rangeof from about 110° C. to about 160° C., or at least 0.15 at atemperature range of from about 130° C. to about 160° C.]. Thus, forexample, the first and second formulations can be selected so as toprovide a desired damping performance at the environmental temperatureat which a printed object is to be used.

For obtaining a desired damping performance, at a selected temperature,first and/or second modeling formulations, differing from one another bya characteristic parameter such as the extent of cross linking of thefirst formulation (expressed, for example, by the relative amount of across linking component (e.g., a multi-functional curable component) inthe first formulation). The selected characteristic parameter canalternatively or additionally be a total calculated Tg of the firstformulation, as calculated by summing individual Tg values of chemicalcomponents of first formulation. The summation is optionally andpreferably a weight sum, wherein each Tg value is multiplied by therelative amount (e.g., weight percentage) of the respective chemicalcomponents of first modeling formulation.

For obtaining a characteristic parameter such as elongation and/ortoughness, first modeling formulations, differing from one another bythe presence or absence of an additional flexible monomer as describedherein, can be used.

The selection of the characteristic parameter(s) can be achieved, forexample, by a look-up table having a plurality of entries, eachincluding a value indicative of the damping (e.g., the tangent of thephase δ) and a corresponding parameter or set of parameters (weightpercentage of the first modeling formulation in the core, extent ofcross linking, total calculated Tg, etc.) corresponding to the damping.The selection of the characteristic parameter(s) can alternatively oradditionally be achieved by one or more calibration curves describing avalue indicative of the damping as a function of the respectiveparameter. Representative examples of such calibration curves areprovided in the Examples section that follows (see, FIGS. 15A-15B).

In various exemplary embodiments of the invention the selection isperformed by a data processor, e.g., data processor 24 or 154. Forexample, the operator can enter, via a user interface, the desireddamping or damping range, and the processor can access a memory mediumstoring a digital representation of the look-up table or calibrationcurve, and display or automatically select the parameter or set ofparameters that provide the desired damping or damping range. Theselection can optionally and preferably be based on the type of modelingformulations that are loaded to the fabrication system (e.g., the typeof modeling formulations in supply apparatus 330), so that the dataprocessor selects only the parameters that are applicable to modelingformulations already loaded into the system. Alternatively, the operatorcan also enter via the user interface, the desired modeling formulation,in which case the data processor selects only the parameters that areapplicable to the modeling formulation that was entered by the operator.

In some embodiments, the modeling material formulations described hereinin any of the respective embodiments, form a part of the above-describedlook-up table.

In some embodiments of the present invention the total calculated Tgvalue of the first modeling formulation is from about 100° C. to about115° C., e.g., about 107° C.

In some embodiments of the present invention the total calculated Tgvalue of the first modeling formulation is from about 120° C. to about135° C., e.g., about 127° C. In some embodiments of the presentinvention the total calculated Tg value of the first modelingformulation is from about 140° C. to about 152° C., e.g., about 146° C.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting. In addition, any priority document(s) of this applicationis/are hereby incorporated herein by reference in its/their entirety.

What is claimed is:
 1. A structure comprises a plurality of layersfabricated by three-dimensional inkjet printing, wherein at least one ofsaid layers comprising: an innermost region formed of a combination of afirst modeling material and a second modeling material interlacedthereamongst; an inner envelope region, at least partially surroundingsaid innermost region, and being formed of said first modeling material,but not said second modeling material; an outer envelope region, atleast partially surrounding said inner envelope region, and being formedof said second modeling material, but not said first modeling material;and a stitching region between said inner envelope region and said outerenvelope region, wherein said stitching region is formed of acombination of said first modeling material and said second modelingmaterial interlaced thereamongst.
 2. The structure of claim 1, whereinsaid inner envelope region completely surrounds said innermost region.3. The structure of claim 1, wherein said outer envelope regioncompletely surrounds said inner envelope region.
 4. The structure ofclaim 1, wherein a ratio between an area of said innermost region thatis occupied by said first modeling material and an area of saidinnermost region that is occupied by said second modeling material isabout 0.25 to about 0.45.
 5. The structure of claim 1, wherein athickness of said inner envelope region, as measured within the plane ofsaid layer, is from about 0.1 mm to about 4 mm.
 6. The structure ofclaim 1, wherein a thickness of said outer envelope region, as measuredwithin the plane of said layer, is from about from about 150 microns toabout 600 microns.
 7. The structure of claim 1, wherein a ratio betweenan area of said stitching region that is occupied by said first modelingmaterial and an area of said stitching region that is occupied by saidsecond modeling material is about 0.9 to about 1.1.
 8. The structure ofclaim 1, wherein said stitching region is devoid of any modelingmaterial other than said first modeling material and said secondmodeling material.
 9. The structure of claim 1, wherein a thickness ofsaid stitching region is less than a thickness of said inner enveloperegion and less than a thickness said outer envelope region, saidthicknesses being measured within the plane of said layer.
 10. Thestructure of claim 1, wherein said thickness of said stitching region isfrom about 70 microns to about 100 microns.
 11. The structure of claim1, comprising a plurality of base section layers forming a base of thestructure, and a plurality of top section layers forming a top of thestructure.
 12. The structure of claim 11, wherein at least one of saidplurality of base section layers and said plurality of top sectionlayers is formed of a combination of said first modeling material andsaid second modeling material interlaced thereamongst.
 13. The structureof claim 11, wherein at least one of a bottommost layer of said top anda topmost layer of said base is formed of said first modeling material,but not said second modeling material.
 14. The structure of claim 11,wherein said bottommost layer of said top is a bottommost layer of astack of layers all made of said first modeling material, but not saidsecond modeling material, and wherein a thickness of said stack measuredperpendicularly to said layers is approximately the same as a thicknessof said inner envelope region, as measured within the plane of saidlayer.
 15. The structure of claim 11, wherein said topmost layer of saidbase is a topmost layer of a stack of layers all made of said firstmodeling material, but not said second modeling material, and wherein athickness of said stack measured perpendicularly to said layers isapproximately the same as a thickness of said inner envelope region, asmeasured within the plane of said layer.
 16. The structure of claim 11,wherein at least one of a bottommost layer of said base and a topmostlayer of said top is formed of said second modeling material, but notsaid first modeling material.
 17. The structure of claim 11, whereinsaid bottommost layer of said base is a bottommost layer of a stack oflayers all made of said second modeling material, but not said firstmodeling material, and wherein a thickness of said stack measuredperpendicularly to said layers is approximately the same as a thicknessof said outer envelope region, as measured within the plane of saidlayer.
 18. The structure of claim 11, wherein said topmost layer of saidtop is a topmost layer of a stack of layers all made of said secondmodeling material, but not said first modeling material, and wherein athickness of said stack measured perpendicularly to said layers isapproximately the same as a thickness of said outer envelope region, asmeasured within the plane of said layer.
 19. The structure of claim 1,wherein said first modeling material is characterized by heat deflectiontemperature (HDT) of at least 90° C., and said second modeling materialis characterized by Izod impact resistance (IR) value of at least 45J/m.
 20. The structure of claim 1, wherein a ratio between elasticmoduli of said first and said second modeling materials is from about2.7 to about 2.9.